Corynebacterium parvum has been administered i.p. to 14 patients with advanced ovarian cancer. Two patients had responded completely to cytoreductive surgery and combination chemotherapy prior to immunotherapy, and one patient with residual disease had received only a single course of C. parvum due to i.p. catheter malfunction. Among the 11 patients with residual disease evaluable for response, from three to eight i.p. treatments with C. parvum produced surgically confirmed tumor regression in five patients (45%) with three partial responses and two complete responses of 5 and 12 months duration. All responders had (a) multiple tumor nodules less than 0.5 cm at the initiation of immunotherapy, and (b) severe abdominal pain and fever after C. parvum injection. Overall, 58 courses of immunotherapy were associated with abdominal pain (91%), fever (67%), nausea (52%), vomiting (31%), and hypotension that responded promptly to i.v. infusion of fluids (10%). Use of i.p. cathethers was associated with two episodes each of infection and intraabdominal bleeding. Administration of C. parvum i.p. has augmented the ability of human peritoneal cells to lyse human ovarian carcinoma cell lines in the presence of specific rabbit heteroantiserum. C. parvum administered i.p. has inhibited the growth of human ovarian carcinoma and may prove useful for modulating the activity of human effectors for antibody-dependent cell-mediated cytotoxicity.