Four squirrel monkeys scheduled to be carried into orbit by the space shuttle Challenger next month were scratched from the flight when they were found to have a form of herpes that could be transmitted to Challenger's astronaut crew.

The monkeys' herpes is not the virus transmitted sexually by humans. It is a virus called Herpes Samirii that is unique to New World primates like the small squirrel monkeys, whose natural habitat is the rain forests of South America.

Nonetheless, the virus is suspected of causing cancer in lower mammals such as rats and therefore is classified as potentially cancerous in humans.

The monkeys with herpes have been dropped from the flight and are being replaced by squirrel monkeys from colonies bred to be virus-free.

Four monkeys recruited from the National Institutes of Health and one from Harvard University have been in training since January for their flight on the second mission of the $1 billion Spacelab, now set for April 29 at the earliest.

"We are still looking for a sixth monkey, even though we may only fly three on the mission," said Dr. Arnauld E. Nicogossian, chief of space medicine for the National Aeronautics and Space Administration.

"One of the monkeys in training is too small and will have to be replaced, and it will be nice to have a backup when the time comes for them to go into orbit," he said.

The monkeys will go into space in cages with their own oxygen, food and water supplies in the Spacelab portion of the shuttle's cargo bay, which is shut off from Challenger's cabin by an airlock.

The four (or three) squirrel monkeys might become members of a monkey group into whose hearts catheters will be surgically implanted and who will ride into space next January to see how their cardiovascular systems respond to weightlessness.

"This flight is kind of a shakedown cruise for the monkeys," Dr. William Thornton, a physician who is a member of the Spacelab crew, said yesterday at Houston's Johnson Space Center.

"The purpose of this flight is to make sure their cages function properly and to make sure they're happy, that they don't get frightened or injured while going up, being weightless or coming down. We want to demonstrate that we can maintain the monkeys in flight."

The seven-day Challenger mission will come on the heels of the scheduled April 12 launch of the shuttle Discovery.

Commanded by Robert Overmyer and piloted by Frederick Gregory, the Challenger crew includes two doctors, Thornton and Dr. Norman Thagard; Donald Lind; Lodewijk van den Berg, a chemical engineer with EG&G Corp. in Goleta, Calif., and Taylor Wang, a physicist with Pasadena's Jet Propulsion Laboratory.