The toxicity of intravenously administered Corynebacterium parvum was observed in 14 patients with stage II melanoma and in 14 patients with advanced ovarian carcinoma. Those with melanoma were rendered disease-free by surgery prior to treatment. The ovarian cancer patients had failed chemotherapy with alkylating agents and were receiving C. parvum prior to chemotherapy as part of an immunochemotherapy trial. Both clinical and laboratory parameters were observed. The mean daily C. parvum dose for melanoma patients was 2.03 mg/m2 and for ovarian carcinoma patients 2.02 mg/m2. The most important clinical toxic effects noted were fever, chills, blood pressure changes, headache, nausea, vomiting and diaphoresis. Laboratory toxicity was mild, with small decreases in hemoglobin levels, white blood cell counts and uric acid and albumin concentrations occurring in some patients. Serum bilirubin and SGOT levels tended to rise. In addition to determining the frequency of clinical toxic effects by treatment course, consideration was also given to frequency per treatment day, correlation of the occurrence of different toxicities in the same patient, time of onset of each toxicity and, for vital signs, to intensity of change and duration. In this analysis no major differences in toxicity were observed when C. parvum was given to the two patient groups.