Properties of 20 coronavirus (CV) isolates obtained from spontaneously infected Papio hamadryas and rhesus monkeys were investigated. Two of them were selected as simian CV prototype strains-CVRM 281 (rhesus monkey) and CVP 250 (Papio hamadryas), and can be offered as candidates to the Coronaviridae family. They are closely related to each other, but differ in some biological properties and polypeptides. These strains belong to the 2nd antigen group of mammalian CV with the prototype strain HCV OC 43 but differ from the latter. The strain CVRM 281 induces experimental CV infection which can be used as a model for investigations of some obscure aspects of human infection. The properties of these viruses suggest their usefulness for diagnostic purposes.