ATLANTA (AP) _ Preliminary results of a study of Vietnam veterans exposed to Agent Orange show few veterans with unusually high levels of dioxin, the potentially harmful chemical in the herbicide used during the Vietnam war, researchers said today.

Blood tests were done on 444 combat veterans, but only one now shows a dioxin level higher than the upper limit for U.S. residents with no known exposure, the national Centers for Disease Control reported.

The CDC report said the median level of dioxin exposure in the veterans was 3.8 parts per trillion, virtually identical to the median level of 3.9 parts per trillion found in the blood of contemporaries who did not serve in Vietnam.

''The distribution of (dioxin) levels for the Vietnam veterans ... suggests that few of the participants in this study have had unusually heavy dioxin exposure,'' the Atlanta-based CDC said, while noting that no threshold level has yet been determined for adverse health effects on humans from dioxin.

Agent Orange, used in Vietnam to destroy jungle plant cover, became contaminated during manufacture with a dioxin that has shown toxic qualities in some laboratory animals. A 1979 law passed by Congress called for a study of the possible health effects on Vietnam veterans from their exposure to herbicides and dioxin contaminants.

The CDC developed the guidelines for its study in 1983, targeting Army veterans who had served in areas of Vietnam heavily sprayed with Agent Orange.

Today's findings are the first from a study of Vietnam veterans who served in the military region around Saigon.

The veterans in the study served in at least one of 65 battalions selected as a result of their duty between October 1966 and March 1969.

Some of the men had been within two kilometers of a confirmed Agent Orange spraying within six days of the spraying at least five times.

The CDC invited 979 veterans to participate, of whom 665 gave blood for the study. Today's results are based on the first 444 veterans' blood samples tested, as well as samples from 75 people who did not serve in Vietnam.

The CDC noted that the dioxin levels reported in the veterans in today's preliminary report ''cannot be generalized with confidence'' to all Vietnam veterans, since the study was not a random sampling of men who served in that war.