In a preliminary and toxicological study, eight patients with carcinoma of the stomach were treated by Corynebacterium parvum immunotherapy prior to surgical excision. Killed C. parvum suspension was injected at multiple sites directly into the tumour mass with the aid of fibre-optic gastroscope at doses of 1.5 mg, 4 mg or 8 mg. Each patient received two injections of the same dose with an interval of 14 days. The second injections were made 14 to 18 days prior to surgery. No chemotherapy was administered during this treatment. Patients were monitored clinically for signs of toxicity. Immunological parameters were assessed on peripheral blood samples taken immediately before each injection and prior to surgery. Antibody titre to C. parvum was estimated by direct agglutination, while cell-mediated responses to C. parvum and 3M KCl tumour extract and phytohaemagglutinin (PHA) were assessed by lymphocyte transformation. Serum carcinoembryonic antigen (CEA) was measured by radio-immune assay. Tumours removed at surgery were examined histologically for the presence of invading immunocytes. These preliminary data will be discussed with respect to the dose of C. parvum administered.