Civilians exposed to radioactive fallout from U.S. atomic bomb tests could have been warned of potential hazards without jeopardizing the nation's development of nuclear weapons, a House subcommittee was told today.

And Atomic Energy Commission officials sometimes veen took a "cavalier approach" to residents claiming fallout damage, the subcommittee was told by Mahlon Gates, the current manager of the Nevada Test Site.

Gates' concessions came under questioning by Rep. Bob Eckhardt (D-Tex.), who had just heard Martha Brdoli Laird tell of the death of her 7-year-old son, Butch, from leukemia in 1957.

"At no time were we ever told about the effects of radiation or what it could do to us," Laird testified. "At no time did they come and test our food and our water, and all this time we were feeding it to our children."

Laird and Gates appeared during a second day of hearings by the House subcommittee on oversight and investigations into the health effects of above-ground nuclear testing on residents exposed to fallout from 1951 to 1962.

In that time, fallout drifted from the test site 102 times. Since then, Gates disclosed today, so-called belowground tests have sent radiation off the proving grounds 31 times.

After the atmospheric testing, childhool leukemia, thyroid cancer and birth defects rose in areas of Utah exposed to fallout, with residents claiming the fallout caused those increases.

No detailed studies of Nevada resident have been conducted yet. But a nuclear physicist disclosed tere that a preliminary survey indicates that the incidence of leukemia in areas of Nevada exposed to fallout is at least as high as and probably higher than would normally be expected.

A major purpose of these hearings is the question of compensating fallout zone residents for what Eckhardt today called "the injustice to the persons sacrified unwittingly." And as Gates indicated, perhaps unnecessarily if people had been warned.

Because radiation-induced cancers look the same as cancers caused by other factors, Eckhardt has said the government may have to compensate all cancer victims in fallout areas to make sure that no fallout-related victims or relatives of victims are left uncompensated. But Donald Gonya, a lawyer with the U.S. Department of Health, Education and Welfare, indicated in a prepared statement today that the administration views the compensating of all cancer and leukemia victims in the fallout areas as "very expensive."

Some 600 claims seeking tens of millions of dollars in damages have been filed with the government by relatives of cancer and leukemia victims, including Laird, alleging the illnesses were caused by fallout.

While the subcommittee is taking a broad look at the health and environmental effects of low-level exposure to radiation, including the recent Three Mile Island nuclear power plant accident, it was Laird's testimony today that drew the most attention and put pressure on Gates. Gates, who joined the Nevada testing program in 1972, nevertheless maintained that his predecessors acted with due concern for public health and safety during a hot war in Korea and a Cold War after that.

But he conceded to Eckhardt, "I agree with you there was no relation of national security to a failure to warn" people like the Lairds, many of whom lived on remote farms and ranches and grew their own food and drank milk from their own cows.

Their gardens and their grazing lands were regularly dusted with fresh fallout during the 1950s.

"There was at times a cavalier approach to sheepherders and ranches by AEC people," Gates said at one point.

Last Thursday in Salt Lake City, sheepmen complained to Eckhardt and Sen. Edwrd M. Kennedy (D-Mass.) in a joint hearing that they were ridiculed by bomb-testing officials when the sheepmen alleged that fallout kicked their stock.

Gates added, however, that he thought there was "serious doubt" htat fallout killed thousands of sheep following a 1953 testing accident, despite evidenc e that AEC officials whitewashed the pursestrings" for more and more tests.

But Laird alleged that the government's attitude was far worse than cavalier. She produced a 1957 letter from a Nevada senator, disclosed last December by The Washington Post, in which fallout zone residents' complaints were dismissed as "communistinspired scare stories."

Laird told, too, of a day in 1957 when two atomic energy officials visited her and dismissed her complaints that her son Butch's leukemia death was the result of bomb tests.

She suggested to them then that for the coming series of detonations, the officials leave their children on her ranch. Laird said that one of the officials replied:

"My God, woman, don't wish that on us."* CAPTION: Picture, REP. BOB ECKHARDT . . . questions test site official