In August and September, 1967, in Marburg, Frankfurt, and Belgrade, there appeared in institutes experimenting with animals serious human illnesses caused by contact with monkeys. Twenty-seven people were affected. Only in Marburg 23 persons were taken ill and, of these, 20 had had direct contact with blood, organs, or cell-derived cultures from monkeys imported from Uganda, of the type known as Cercopithecus aethiops. Three patients were infected through contact with people already sick. The period of incubation, which could be accurately determined in patients who had had only a single contact with infectious material, was of five to seven days’ duration.