Tests of American diplomats and families who have served in Moscow show temporarily abnormal white blood cell counts, though no sign of serious disease, in a third of those tested, the State Department disclosed yesterday.

The test were made to seek possible ill effects of the Soviet microwave radiation that has been beamed at the U.S. embassy since the early 1960s. The radiation came from electronic equipment aimed at the embassy.

But the study shows "no connection" at all between the blood counts and the microwaves, the department said.

Dr. Herbert Pollack, State Department medical consultant, said he could be "absolutely sure" of this because persons heavily exposed to the microwaves have counts no different from those who were never exposed.

"We're dealing with some unknown factor in the Moscow environment," he said in an interview. "I don't know what it is, but some kind of viral infection seems most likely. It could be low-grade upper respiratory infection. A more remote possibility is some parasite, like giardia" - a drinking water organism known to cause temporary intestinal problems in visitors to Leningrad.

Pollack said Americans stationed in the Soviet Union often visit Leningrad, "and we're making further studies" to find out whether the organism could be present in Moscow, too.

Cancer - including leukemia and other blood malignancies - are among the most feared results of long-term exposure to microwave rays. Pollack said no cancers or blood illnesses "ascribed to microwave radiation" have been found in present or past Moscow personnel, despite some reports of such illnesses in newspapers and magazines.

"There have been some malignancies, just as in personnel who serve in other countries," Pollack said. But he added that the final answer to whether the microwaves have caused any disease will have to await a new Johns Hopkins University study - a study the State Department commissioned - of 3,500 ex-embassy workers.

Pollack called the elevated blood counts "no cause for alarm." This is partly because "they are not very high, and ordinarily a doctor wouldn't worry about them," he said, and partly because "they are reversible, they return to normal" two weeks to a year after those affected leave Moscow, "whereas the blood changes caused by leukemia or malignancies are not reversible."

Two medical specialists consulted by The Washington Post agreed that the figures Pollack supplied showed no cause for alarm.

The two - Dr. Edward P. Radford, professor of environmental medicine at Johns Hopkins University, and Dr. Paul McCurdy, Washington area Red Cross blood director - agreed too that the most plausible explanation for the effects described is either some infection or parasite.

But they also said that, as doctors, they would want to keep following the people concerned - "probably twice a year for a year or two, then once a year unless they developed some symptoms or problems," McCurdy said.

"The truth is that we don't really understand much about the effects of microwaves, and here's a group of whom at least some had microwave exposure and we should follow them," Radford said.

The State Department will continue doing so, Pollack said, and Johns Hopkins will trace the medical histories of some 3,500 former Moscow personnel and compare them with the histories of 3,500 foreign service officers who served elsewhere.

The Washington announcement - and simultaneous invitation to "U.S. citizens living in Moscow" to come to the embassy for blood tests - comes a year after the Soviet radiation was disclosed.

There have been reports that the Soviets use the rays, which still continue, either to disrupt American eavesdropping or to eavesdrop on the Americans. U.S. officials have refused to say why they think the Soviet Union beams microwaves at the embassy, though two months ago they designated it an "unhealthful post" after first refusing to do so.

The State Department yesterday reported only "slightly higher than average numbers of lymphocytes" - one kind of white blood cell - in some persons.

Pollack supplied figures showing that in 213 present and past Moscow personnel and dependents the average lymphocyte count was 40 per cent higher than the count in other foreign service personnel; that the also important neutrophil (another kind of white cell) count was 15 per cent higher, and that there were also higher counts of two other white cells, eosinophils and monocytes.

He said the figures were equally high in several groups: those who served on embassy floors that got the radiation; those got more radiation after the Soviets boosted the level in May, 1975; those who got less after the Soviets reduced it last February and the embassy installed anti-radiation screens, and those who got no radiation at all.