Human monocytes activated in vitro by lymphokine-containing supernatants of autologous or allogeneic lymphocytes stimulated in vitro by Corynebacterium parvum (CP) [Propionibacterium acnes] expressed increased ability to suppress DNA-synthesis in a human [NHiK 3025 cervical carcinoma] tumor cell line. Monocyte activation was not dependent on in vitro differentiation of monocytes, enhanced cytostatic ability being observed at all stages of in vitro differentiation. The lymphokine-induced cytostatic ability was not affected by intensive washing and trypsin treatment of the activated monocytes, but disappeared during 48 h of in vitro culture of the activated cells. The increased cytostatic ability of lymphokine-activated monocytes did not seem to be due to stable supernatant factors released from monocytes. CP stimulated DNA-synthesis in peripheral blood lymphocytes of 28 normal donors, thus confirming the mitogenic effect of CP on human lymphocytes. Lymphokine production in response to CP correlated with the magnitude of DNA-synthesis, but appeared before DNA-synthesis could be detected in the lymphocytes.