Little is known of the localisation of injected Corynebacterium parvum. We therefore developed a method of radiolabelling this vaccine and determined its distribution in the mouse. Live C. parvum (Wellcome, strain CN 6134) was grown in the presence of 3H-thymidine and subsequently killed by formalin. A high activity, 1-6 x 10(5) cpm/0.1 ml of a 7 mg dry weight/ml concentrated suspension, was obtained. Its biological properties (hepatosplenomegaly and antitumour effects) were similar to those of commercially available vaccine. After intravenous or intraperitoneal injection into normal mice, high activity was recovered in liver and moderate activity in spleen, lungs and small gut. In contrast, after subcutaneous injection, most activity was recorded at the injection site, and little in other tissues. A similar distribution of labelled C. parvum was found in tumour-bearing mice. Only moderate counts were detected in tumour.