When Advanced Cell Technology, a small biotechnology company in Worcester, Mass., announced on Sunday that it had taken the first steps in producing human embryos through cloning, it could not report lasting success; all the embryos it created had died.

It could not even report that it had used groundbreaking techniques; its methods had already been used in animals.

Some scientists even suggested that what the company was doing was not cloning at all.

But if there is a future in human cloning, either for reproductive purposes or to create cell lines for use in treating diseases, people may one day say it started in Worcester.

Despite the storm of protest that the company's announcement has provoked, that would be just fine with Advanced Cell Technology. Its president, Dr. Michael D. West, says the company feels pressure to keep the world informed about what it is doing in so controversial a field. But he concedes that the desire to be the first to claim to have created a human embryo by cloning was a factor in the company's decision to publish its results so far.

Whatever the scientific significance of Dr. West's announcement, its political significance was profound. President Bush denounced the work as immoral, and there were loud calls for Congress to outlaw it.

Shadowing the raging dispute on whether such work should be outlawed is a major scientific question: Is the human-cloning attempt a milestone or a forgettable blunder? The answer, cloning experts say, is that it is impossible to know.

Work with animals has shown that cloning is something of an art. There are no rules or formulas. Success, when it comes, can be unpredictable and nearly inexplicable. It could be that human cloning is extraordinarily difficult and that it will take years and thousands of attempts to make it work. Or it could be that a simple change in the laboratory procedure will turn failure into success. That has been the experience of scientists who work at cloning animals.

For Advanced Cell Technology, these uncertainties loom large. The company is betting that it can perfect human cloning, creating embryos not for reproductive purposes but as a source of stem cells. Human embryonic stem cells could, in theory, grow into any of the body's tissues and organs, and the company wants to provide them as replacement cells to patients suffering from any of a wide variety of diseases.

The small company has a track record of achievement in the world of cloning animals; some of the leading cloning researchers are on its payroll.

But it also has a track record of astute dealings with the news media. In interviews, Dr. West acknowledged that scientists for the company had published their results in a little-known online publication - E- biomed: The Journal of Regenerative Medicine - because E-biomed had agreed to arrange for distribution to coincide with articles in Scientific American and U.S. News and World Report.

Like many other small biotechnology concerns, privately held Advanced Cell Technology attracts investors with promise, not profits. And though Dr. West said the company had just completed a round of fund-raising, he noted that it would have continuing needs for money to finance its work.

"We're going to require hundreds of millions in investments," he said, "before we become profitable."

In the work reported on Sunday, the company's scientists, led by Dr. Jose Cibelli, used a standard technique that involves taking the genetic material out of an unfertilized egg and inserting in its place the DNA of an adult cell. In theory, the egg then uses the genes from the adult cell to direct its development, turning into an embryo that is an exact genetic copy of the donor of the adult cell.

The company tried to clone with two types of adult cells: skin cells and cumulus cells, which are cells that cling to human eggs. The researchers added skin cells to 11 eggs; none divided even once. They added cumulus cells to eight eggs; three divided once or twice, the others not at all.

Stem cells appear only after an embryo grows for about five days and, more important, forms a blastocyst, a sphere of cells with a ball of stem cells inside it. The Advanced Cell Technology embryos that were created by cloning were not even close to that developmental stage.

Dr. Ronald M. Green, a Dartmouth professor who heads the company's ethics board, says he prefers not even referring to the cells as embryos. He would like to call them "cleaving eggs," he said.

In fact, scientists say, eggs can divide a few times without making any use of their genes, so the fact that a few eggs divided a few times does not at all mean that the
goal of the experiment - to add a new set of functioning genes to an egg - was even close.

But cloning failures can suddenly turn to successes, as those who have cloned other species attest.

That was the experience of Dr. Randall Prather, a cloning expert at the University of Missouri, in years of efforts to clone pigs. Over and over again, Dr. Prather would start the cloning process, and then the cells, like those in the Advanced Cell Technology study, would simply die.

Now he and others can clone pigs, but he does not know which changes in his laboratory procedures made the difference. All he can say, Dr. Prather remarked, is,
"Yeah, now it works."

Cloning also depends on scientists' having a delicate touch, experts said.

One scientist now with Advanced Cell Technology, Dr. Tony Perry, who worked on mouse cloning experiments at the University of Hawaii, said it took endless hours of practice to do the careful manipulations of microscopic cells involved in cloning. Some people develop a feel for the work, while others, no matter how hard they try, are never very good.

"It requires a kind of eye-hand coordination" and constant practice, Dr. Perry said, recalling months of practice, seven days a week, 10 hours a day. "If you lapse in your practice for two weeks," he said, "you don't return to point zero, but you're a little bit rusty."

There are also puzzling and unpredictable differences between species. Dr. Ryuzo Yanagimachi, who cloned the mice with Dr. Teruhiko Wakayama, also now with Advanced Cell Technology, said about 2 to 3 percent of efforts to clone cattle resulted in the birth of a live animal. Most of the rest die very early: only about 20 percent of the embryo clones make it to the blastocyst stage.

With mice, Dr. Yanagimachi said, about 50 to 60 percent of the embryo clones make it to the blastocyst stage. But even more die afterward. In the end, he said, the same percentage of mouse cloning attempts succeed as cattle cloning attempts.

Given that the human cloning work ended in failure, some, like Dr. Steen Willadsen, a cloning pioneer in Windermere, Fla., have asked why Advanced Cell Technology even bothered to publish its results, orchestrating at the same time a media blitz.

In theory, Dr. Willadsen said, the publication will offer other researchers clues about "things not to do." But, he added, the crucial unknown detail that doomed the attempt may not be so obvious, and may not even appear in the paper. "There might be some trivial thing that is standing in their way," he said.

"All one can say," Dr. Willadsen added, "is it didn't work this time."