SPACE CENTER, Houston (AP) _ Two monkeys, instead of four, will fly on the space shuttle next month because of fears the astronauts could be exposed to a form of simian herpes.

Robert Overmyer, a commander of the 51-D Spacelab mission set for launch April 29, said at a briefing Tuesday that NASA was insisting on flying germ- free monkeys to avoid the ''remote'' chance of infecting the seven-man crew with herpes.

Four squirrel monkeys originally were planned for the seven-day science mission but NASA scientists discovered the animals were infected with herpes virus samarai.

Most experts consider this virus to be non-infective for humans but an extensive NASA study of monkey handlers uncovered proof that the virus did in fact transfer from one species to another.

''I feel very comfortable with our position to fly pathogen-free monkeys,'' Overmyer said. ''We don't know how it (the virus) would react in zero gravity.''

NASA is flying only two monkeys, leaving two other cages empty, the mission commander said, because they were unable to find four monkeys hardy enough for a space flight and free of herpes.

The monkeys are being flown to test a new cage that will be used in future space flights to house animals. Also aboard the flight will be 24 rats.

Dr. Bill Thornton, a physician-astronaut who will care for the monkeys in orbit, said the chances were remote that the astronauts would contract herpes but he thought the management decision ''was wise.''

Thornton said squirrel monkeys, which are native to the Americas, were chosen because they are not susceptible to an even more serious virus that could be fatal to man.

This virus, called the Monkey V, takes up to 20 years to incubate following exposure and is common in African and Asian primates.

Overmyer said he has instructed the crew that the two monkeys are not to be named and are to be regarded strictly as laboratory specimens.

''Besides, they aren't very pleasant,'' he said. ''They bite and they're not lovable at all.''

Thornton, who will have the primary responsibility for caring for the animals, agreed. He said in his first introduction to the two-pound primates, one of them bit the thumb of a laboratory technician to the bone.

Thornton said when he tried to examine the animal with a stethoscope, it bit through the tube of the instrument, narrowly missing Thornton's face.

Others crewmen on the Spacelab mission include pilot Frederick Gregory; mission specialists Don Lynd and Dr. Norman Thagard; and payload specialists Taylor Wang and Lodewijk van den Berg.

Wang will operate his own experiment, a study in fluid dynamics. Van den Berg will experiment with growing crystals in weightlessness. Lynd will conduct a study of the aurora over Australia in the first attempt to record the natural light phenomena from space.

The mission will be launched from the Kennedy Space Center and return there after a week in orbit. Overmyer said plans are for the Spacelab mission to be launched 17 days after the flight of 51-B, the much-delayed mission that includes Sen. Jake Garn, R-Utah, among the crew.

Overmyer said tentative plans now are for the earlier mission to be launched on April 12. The commander said, however, that a delay in 51-B would also delay the Spacelab flight on a day-for-day basis.