Lymphocyte-mediated cytotoxicity (cell-mediated immunity) to ovarian carcinoma cells and serum blocking factor were measured in 37 patients. Short-term cultures of tumor cells and a low ratio of effector to target cells were used throughout the study, minimizing nonspecific cytotoxicity. Sixteen patients were followed for long periods of time and correlation with the course of the disease and with therapy could be obtained. Although the level of cell-mediated immunity did not always correspond to the clinical status of the patient, the presence of blocking factor was associated with clinical relapse in 14 of 16 patients. Chemotherapy with single alkylating agents or combinations of drugs caused no significant or permanent depression of cell-mediated immunity. Blocking factor disappeared in 2 patients during remission. The chemotherapy for ovarian carcinoma may not be significantly immunosuppressive against established levels of cell-mediated immunity and may (in certain instances) have effects potentially beneficial to the host as evaluated by lymphocyte-mediated cytotoxicity and blocking factor studies.