We studied the response of young squirrel monkeys (Saimiri sciureus) to experimental Herpesvirus saimiri (HVS) infection. Three groups of monkeys without latent HVS infections were treated as follows: One group was experimentally infected with HYS, one was caged with the inoculated monkeys, and the third was isolated from the other monkeys. HYS was first isolated from peripheral lymphocytes of the infected monkeys at 17–24 days post inoculation (PI) and from their cagemates at 60–78 days PI. About the same time that virus was isolated, the animals developed HYS serum antibodies. Clinical observations and hematologic studies did not indicate HYS-induced disease in any animal. During the 5-month observation period, the isolated monkeys remained free of infection. We failed to isolate HVS from their peripheral lymphocytesi and isolated monkeys did not become seropositive. This proved what was assumed from previous epidemiologic studies, that HVS is transmitted horizontally and not vertically in squirrel monkeys.