WASHINGTON (AP) _ Large-scale research cannot be conducted on the effects of Agent Orange because exposure to the defoliant was not widespread among Vietnam veterans and only a few of them show significant amounts of it in their blood, a congressional agency said Wednesday.

A staff report by the Office of Technology Assessment agreed with findings by the Centers for Disease Control, which found significant levels of dioxin, a key ingredient in Agent Orange that is believed to cause cancer, in the blood of just two of the 573 Vietnam veterans tested.

''All the existing data support the fact that most ground troops in Vietnem did not have heavy Agent Orange exposure, and that those who might have would be exceptions,'' the report said.

''This ... study confirms a low probability for meaningful Agent Orange exposures for Vietnam veterans as a group,'' the OTA report said. ''Within the group of veterans included in the validation study ... we would not expect that the contribution of exposure from sprayings in Vietnam could be distinguished from other exposures that these veterans might be expected to have had in the normal course of life.''

The OTA report issued this month said if any study of Agent Orange is considered, it should be narrowly focused.

The report acknowledges that its conclusion runs counter to the widely held belief that most of the ground troops in Vietnam were heavily exposed to the chemical.

Among those who disagree with the conclusion is Tony Principi, chief minority counsel for the Senate Veterans Affairs Committee who served in the Navy in Vietnam stationed along the Vam Co Dong River.

''We dumped an awful lot of that crap in Vietnam,'' Principi said Wednesday. ''The scientists were not out in the jungles or on the rivers.''

The CDC study also found no overall difference between tests for Vietnam veterans and veterans who did not serve in Vietnam.

However, a group of 30 people whose direct exposure to Agent Orange had been verified had ''much higher serum dioxin levels'' than the larger group of Vietnam veterans, the study said.