Poxviruses were isolated from the kidneys of an outwardly healthy chimpanzee trapped in an area of the Democratic Republic of the Congo where a case of monkeypox had recently occurred in man, from the kidneys of clinically healthy cynomolgus monkeys in a colony in the Netherlands, and from monkeys suffering from monkeypox during outbreaks in colonies in the USA. It was established that two of the three viruses isolated from animals asymptomatically infected-namely, strain Chimp-9 from the chimpanzee and strain 64-7255 from the cynomolgus monkeys-although similar to one another differed markedly from the classical Copenhagen strain of monkeypox virus. These two viruses were characterized by the formation of small, monomorphic, well-defined pocks without haemorrhages on infected chick embryo chorioallantoic membrane, by the small plaques of the proliferative type that they produced in cell cultures, by the absence of reactions when they were applied to scarified rabbit skin and the absence of marked necrosis when they were inoculated intradermally into rabbits, by their intensive replication in pig embryo kidney cell cultures, and by a number of other features. It is therefore possible to describe both the viruses as being very close to the variola virus. The Chimp-9 and 64-7255 strains differed from the variola virus only in their greater pathogenicity for white mice after intracerebral inoculation. The other virus isolated from a symptomless cynomolgus monkey-strain 64-9411-resembled the two viruses isolated from monkeys suffering from monkeypox and did not differ from the Copenhagen strain.