``I do not support research from aborted fetuses,'' he said.
He did not say whether he would move to block federal research funding - an act that many scientists say could stop promising research into therapies for numerous diseases. Aides said afterward he was signaling his intent to do so.
Bush had indicated his opposition to such research during the presidential campaign, but the remarks Friday were his first on the topic since taking over the White House a week ago.
``I will let you know when I decide all policy decisions, but the answer to your question is no,'' Bush said when asked whether he believes federal money should be spent on fetal-tissue and stem-cell research from abortions.
On Monday, two days into his presidency, Bush had moved to restore restrictions on U.S. foreign aid to family-planning organizations involved in abortion. Anti-abortion conservatives were a cornerstone of his political campaign.
Scientists say research with stem cells, master cells that are the building blocks for all other tissue in the body, could lead to revolutionary treatments for problems from Alzheimer's to paralyzing spinal cord injuries.
Stem cells can come from aborted fetuses or from embryos left over from fertility clinics. A few also can be found from adult tissues, but scientists say fetal and embryonic stem cells are the most flexible and thus most usable.
Bush did not specifically address embryonic stem cells.
Some conservative groups oppose using embryonic stem cells for research because culling them kills the embryos. Scientists now know how to multiply embryonic stem cells in laboratories without killing additional embryos. Unless Bush intervenes, the National Institutes of Health plans to begin funding research with just those lab-grown embryonic stem cells as early as this spring.
Bush has said in the past he supports an alternative method using fetal tissues retrieved from miscarriages. But scientists say such tissue is seldom usable because of genetic abnormalities in the fetus that caused the miscarriage.
``I believe there's some wonderful opportunities for adult stem-cell research,'' Bush said. ``I believe we can find stem cells from fetuses that died a natural death, but I do not support research from aborted fetuses.''
He commented in a question-and-answer session during a meeting with Democratic and Republican governors.
Shortly before Bush took office, his spokesman refused to address whether the new Republican administration would shut down government research on the stem cells of discarded human embryos.
Press secretary Ari Fleischer, quoting his boss' statements during the campaign, said Friday that Bush ``would oppose federally funded research for experimentation on embryonic stem cells that require live human embryos to be discarded or destroyed.''
But Fleischer, questioned by reporters, would not say whether Bush intends to block the NIH, which is now accepting grant applications for research on lab-grown embryonic stem cells initially harvested by private researchers.
Bush stopped short Friday of saying whether or how he might block the NIH funding.
Incoming Health and Human Services Secretary Tommy Thompson has likewise sidestepped the issue. But as an anti-abortion governor of Wisconsin, Thompson praised as medical pioneers the University of Wisconsin scientists who first grew embryonic stem cells in their laboratory.
The White House is reviewing all rules and executive orders implemented by the Clinton administration, including those on abortion-related research.