Peripheral lymphocytes from white-lipped (Saguinus fuscicollis and S. nigricollis) marmosets infected with Herpesvirus saimiri (HVS) were separated from whole blood on Ficoll-Hypaque gradients. The cells were cultivated in vitro in suspension or with susceptible vero cells (a continuous line of African green monkey kidney cells) and were examined for infectious HVS and HVS-associated antigens. HVS antigens were not demonstrated in smears of lymphocytes immediately after removal from the marmosets, but lymphocyte smears examined after 24, 48, and 72 hours' cultivation showed that 1–5% of the lymphocytes contained stainable HVS antigens. Infectious HVS was not recovered from super-natants or cell-free extracts of lymphocyte suspensions after cultivation up to 120 hours, and herpes-type particles were not seen by electron microscopy in lymphocytes cultured up to 72 hours. HVS antigens developed in lymphocytes 24 hours after cocultivation with vero cells, and already a small percentage of lymphocytes had transferred the HVS genome in some form to the vero cells. HVS was recovered from supernatants and cell-free extracts of cocultivation cultures 48–72 hours after incubation.