The lymphocytes of five owl monkeys infected with Herpesvirus saimiri (HVS) and three control monkeys were studied. Lymphocytes were separated on Ficoll-Hypaque gradients, incubated in suspension cultures, and prepared for electron microscopic and immunofluorescent study at 24, 48, and 72 hours after the beginning of culture (AC). Buffy coats of whole blood immediately after bleeding were also prepared for study. At the time of the study, four of the five infected monkeys had died of malignant lymphoma and lymphocytic leukemia. HVS virions were demonstrated by electron microscopy and HVS antigens by immunofluorescence in 1 to 20 per cent of the lmyphocytes from infected monkeys in two of five cultures at 24 hours AC, four of five at 48 hours AC, and all five at 72 hours AC. There was quite good agreement between the electron microscopic and the immunofluorescent data. None of the control monkey cultures and none of the buffy coat preparations contained HVS virions or antigens. By electron microscopy the great majority of the virus particles were nucleocapsids within the nuclei of lymphocytes. Very seldom were enveloped virions observed. There was some evidence to suggest that the higher the percentage of lymphocytes containing HVS the poorer the prognosis for the monkey. HVS was isolated from all five infected monkeys by cocultivation of lymphocytes with Vero cells. Control lymphocyte cocultivations were negative.