A cell-associated herpesvirus was isolated from macaque monkeys suffering from a disease similar to varicella. The agent can be differentiated from Herpesvirus varicellae by its distinct CPE, its inability to replicate well in human fibroblasts, and its antigenic composition. The virus appears distinct from herpesviruses previously isolated from monkeys with vesicular exanthems in its characteristics of growth in tissue culture and in the low case-fatality ratio of the associated disease. Epidemiologic evidence indicated that the virus originated in Southeast Asia. The disease may serve as a model in animals for study of human infections with Herpesvirus varicellae.