Medical examinations have revealed a 75 percent increase in the past year of radioactive cesium in the bodies of Marshall Islanders living on Bikini Island, site of the fallout from a 1954 U.S. hydrogen bomb test.

Government officials described as "incredible" the increased level of cesium, which alone is causing individual internal radiation almost two thirds the allowable U.S. level for absorption from internal and external sources.

The new findings, from medical examinations in April, suggest that the 139 men, women and children living on Bikini are well beyond the overall radiation safety limits set by U.S. government scientists for exposure in the general population.

Cesium is only one of the radiation sources to which the Bikinians have been exposed. Strontium 90, another ingredient of fallout, is in their bodies but has not recently been measured. Yet another source of exposure is external radiation from fallout material which has remained in the ground since the 1954 test.

Officials expect that the overall average radiation level will have doubled in the past 12 months when they complete all current tests.

The Marshallese have been ordered to stop eating coconuts and other local fruits grown in the still-contaminated Bikini Island soil. For the past six months all their food and drink has been supplied from outside, except for fish that is considered safe to eat.

The only exception, officials said, occurred during a drought period when the islanders turned to coconut milk to satisfy thirst.

Despite the precautions, April examinations of 99 adults and children over 5 uncovered 12 with radioactive cesium in their bodies above the level considered safe over the long term by U.S. standards.

The tests on Bikini were primarily done with a device called a whole body counter, which measures radiation from cesium but not strontrium. Strontium 90 is measured by urine analysis.

Department of Energy officials who have reviewed the current data believe that when the overall body burdens, including strontium, are calculated, levels will be even higher than those now being reported.

Cesium is attracted to muscle tissue and strontium adheres to bones when they get inside the human body. The radiation these elements then produce affects bone and tissue cells, raising the risk of cancer if the process continues.

Cesium inside a human body drops off by one-half in about 100 days, scientists believe, if more is not interested. Strontium, however, lasts much longer - up to 30 years before half is gone.

Experienced government scientists said over the weekend that the Bikinians may have ingested into their bodies the largest amount of measurable radiation of any population.

Many of those examined last month had returned to Bikini Island in 1969 after it was declared safe for hibitation by the Atomic Energy Commission.

Officials who have reviewed the April medical findings don't, as one put it, think this is a life-threatening situation" requiring immediate evacuation of Bikini by all 139 residents.

They do believe, however, the danger to children is greater than to adults since their smaller bones and organs are more easily damaged by the internal radiation caused by the ingested radioactive elements.

One child, for example, was recorded in April as only slightly below the allowable standard with cesium alone measured.

"If this occurred with one of our facilities," a DOE official said, "we'd move the people as soon as we could."

A suggestion has been made to Interior Department officials, who administer Bikini as part of a U.S. trust territory, that children be taken off the island immediately and that adults follow within several months.

Late yesterday, it was learned that Interior officials have decided to move the Bikinians temporarily to another Marshall Island atoll within 75 to 90 days.

This morning, the officials will appear before a House Appropriations subcommittee to report on the Bikini medical situation. Interior is asking the subcommittee for $15 million to relocate not only the 139 Bikinians on the island, but also the 400 Bikini people who now live on Kili Island.

The 539 together make up that total Bikini population that has sprung from the 120 people who were moved off Bikini atoll in 1946 to make way for the first series of Pacific nuclear weapons tests.

Leaders of the Bikinians on Kili are expected to tell the subcommittee today that they want their fellow Marshallese moved off immediately. That may bring out the fact that, according to officials who have recently visited Bikini, the residents still don't want to leave.

In 1969, when Bikinians were first permitted to return to their island, AEC officials were quoted as saying, "We can't say there is absolutely no radiation danger, but if there is we can't find it."