Antiabortion activists said yesterday they would boycott American Express and 43 other companies that donate funds to Planned Parenthood.

Leaders of the Christian Action Council said the move follows a letter-writing campaign in which companies were asked to withdraw support from Planned Parenthood because abortions are performed at its clinics.

The organization said the businesses should "stay out of and keep out of the abortion business."

The Christian Action Council said it was focusing its first boycott attempt on American Express because that company is widely known, not because it donated the most money.

American Express spokeswoman Toni Maloney said the firm has not taken a stand on abortion.

"We give to a lot of various projects that basically meet the themes of community service and education," she said. "We gave two modest grants to Planned Parenthood projects in 1989 and we felt that both of those fell into those categories."

Maloney said American Express had not yet made a 1990 grant to Planned Parenthood. "But this year is only half over and last year's donations were made late in the year," she said.

David Andrews, vice president of Planned Parenthood of America, said of the boycott threat, "American Express is a very sophisticated corporation and they're not going to be pushed around by the CAC."

The CAC said its actions led American Telephone & Telegraph Co. to withdraw its support for Planned Parenthood, beginning in 1991, and that at least 10 other companies had followed AT&T's lead.

"Letters were sent pointing out to AT&T that it was, in a sense, shooting itself in the foot by supporting an agency that kills its future customers," said Joe Scheidler of the Pro-Life Action League.

Andrews said he knew of no other businesses that had withdrawn financial support from Planned Parenthood.

"I think that AT&T's withdrawal of support backfired not only on AT&T but also on the CAC. We got thousands of new donors after that," he said. "Most corporations don't appreciate any interest group like the CAC trying to dictate their corporate support."

The CAC said companies will be removed from its boycott list once they pledge to withdraw support or if support has not been given for more than five years.