A test for monkeypox-specific antibodies is described. Monkeypox immune sera can be made type-specific by immunoabsorption with heterotypic poxvirus extracts. Monkeypox-specific antibodies were demonstrated in sera from 9 cynomolgus monkeys (Macaca fascicularis) that had previously been experimentally infected with monkeypox. Monkeypox-specific antibodies were found in 3 wild-caught African monkeys (Cercopithecus spp.) and in 3 human sera collected from Africans in the Ivory Coast and Nigeria 3(1/2)-4 years after they had suffered a pox-like infection. Monkeypox had been recognized in one of the patients by virus isolation, and had been suspected in the others for epidemiological reasons. Vaccinia-specific antibodies were found in 4 human sera collected 6 weeks after smallpox vaccination.The serological results provide the first laboratory evidence of a monkeypox reservoir in wild monkeys.