The effect of Herpesvirus saimiri (HVS) infection on the response of owl monkey lymphocytes to general mitogens was examined during the development of neoplastic disease. The reactivity of the lymphocytes was then correlated with the clinical condition of the infected monkeys and the content of virus rescued from the peripheral blood lymphocytes (PBL). Eight monkeys developed lymphoma which, in six monkeys, was accompanied by lymphocytic leukemia. All animals that died of HVS-induced neoplasia consistently showed a lack of mitogenic response to phytohemagglutinin and concanavalin A. The response to pokeweed mitogen, while always reduced, was generally less markedly affected than the response to the other two mitogens. Lymphocytes from five of the leukemic animals demonstrated an elevated level of spontaneous DNA synthesis in culture late in the disease. This increased spontaneous DNA synthesis tended to correlate with the rescue of HVS from the PBL as demonstrated by the infective center assay. Although mitogenic hyporesponsiveness corresponded with HVS rescue from PBL in six of nine monkeys, the impairment of normal lymphocyte responsiveness sometimes preceded virus recovery.