A medical consultant who had won $2.6 million in federal contracts and a former official with the U.S. Agency for International Development (AID) pleaded guilty yesterday to charges of bribery and conspiracy for rigging contracts to supply nonlethal aid to the Nicaraguan contras.

George F. Kraus, president of New York-based Kraus International, appeared before Judge George H. Revercomb in U.S. District Court and pleaded guilty to bribing an undercover AID official.

Kraus agreed to cooperate in the government's investigation after an undercover agent videotaped him handing over cash last January for confidential information, according to court documents.

John Massey, a former AID official who has been living in Guatemala but has a residence in San Francisco, according to the documents, also pleaded guilty to receiving more than $11,000 from Kraus in return for insider information. Massey drafted and reviewed contract proposals while with AID, from 1987 to 1989, and later served as a consultant to the agency without disclosing his ties to Kraus.

Both are scheduled to be sentenced in September. Kraus faces maximum imprisonment of 15 years; Massey faces up to five years. Each can be fined $250,000.

According to court records, the investigation began after the AID office in Honduras received a tip in 1988 that contracts to supply contras with nonlethal aid were being fixed. After being taped offering an undercover agent a bribe, Kraus agreed to cooperate for a lighter sentence and implicated Massey and Donald F. Enos, the number two AID official with the contra project.

In May, Enos, 47, was sentenced to a year in prison and fined $5,000 for taking $93,000 in bribes from Kraus.