Between October 1970 and May 1971, six cases of human infection with monkeypox virus were identified in Liberia, Nigeria, and Sierra Leone. Four of the cases were confirmed by virus isolation and two were diagnosed on the basis of epidemiological and serological investigations. All the cases occurred in unvaccinated individuals.Post-infection serological studies showed high haemagglutination-inhibition and neutralizing titres to pox group virus in four of the cases. Repeated challenge vaccination of all cases with potent smallpox vaccine resulted in equivocal reactions.In all, 24 susceptible household contacts were exposed to the infected cases, but none developed disease. All the contacts subsequently responded to vaccination with a primary reaction, thus confirming their susceptibility and ruling out asymptomatic infection.