Gov. Gray Davis signed legislation Sunday that expressly permits the research, which has been strongly opposed by anti-abortion groups and the Roman Catholic church because it involves the use of fetal and embryonic tissue.
The issue captured headlines more than a year ago when President Bush restricted federal funding for human embryonic stem cell research to a select number of existing cell lines.
Supporters of the California legislation say the law will attract scientists who someday may be able to cure chronic diseases through the research. Proponents include actor Christopher Reeve, who has been a stem cell research activist since an accident left him paralyzed from the neck down. He believes stem cell research could help treat paralysis.
"Since stem cells were first isolated in 1998, the political debate has had a chilling affect on our scientists," Reeve said Sunday. "It is painful to contemplate what advances could have been made" if that research wasn't stifled.
Stem cells, which are found in human embryos, umbilical cords and placentas, can divide and become any kind of cell in the body. Opponents contend the research is tantamount to murder because it starts with the destruction of a human embryo.
On board Air Force One as President Bush flew to New Jersey on Monday, White House press secretary Ari Fleischer initially chalked up the California law to states' rights.
"The president has always said states have authority within their states," Fleischer said.
Later, Fleischer amended his remarks.
"The president thinks that all policies — state or federal — need to promote a culture that respects life and, in that, he does differ from what California and the governor there have done," he said.
State Sen. Deborah Ortiz wrote the bill that states that California will explicitly allow embryonic stem cell research, and allows for both the destruction and donation of embryos.
The bill requires fertility clinics that do in-vitro fertilization procedures to inform women that they have the option to donate discarded embryos to research. It requires written consent and bans the sale of embryos.
Ortiz and supporters of her bill said the research could be valuable in curing or alleviating chronic and degenerative conditions, such as Parkinson's disease, Alzheimer's and spinal cord injuries.
The law will attract "the best and the brightest" researchers to California and halt the migration of stem cell researchers to other countries where it is permitted, said Larry Goldstein, a professor at University of California, San Diego.
Movie producer/director Jerry Zucker joined Davis and Reeve in the announcing the law, saying he learned about stem cell research after discovering that his young daughter had diabetes.
"After learning the daily routine, we began to ask what was being done to cure diabetes," he said. "Everyone told us that embryonic stem cell research is her best hope for a cure."
Zucker said he immediately discovered "that the biggest obstacle in finding a cure for our daughter is our own government."
Congress hasn't acted on any stem cell research bills, or a bill to ban human cloning, and Ortiz said there was still a question over whether California's law would be pre-empted by a federal statute.
Measures pending in Congress range from allowing research to criminalizing it and prosecuting those who traveled abroad for treatment derived from stem cell research.
Davis has signed another bill which makes permanent a temporary ban on human cloning for reproductive purposes, said his spokesman Steve Maviglio. That ban was set to expire at the end of the year.