The relative effectiveness of local irradiation alone or combined with Corynebacterium parvum (C parvum) treatment has been investigated employing four tumors: a mammary carcinoma (MCa) (nonimmunogenic), a fibrosarcoma (moderately strongly immunogenic), and two squamous cell carcinomas (SCC-2 being weakly and SCC-4 being very weakly or nonimmunogenic). C parvum treatment was started when the isotransplanted tumor growing in the mouse leg was 5 mm in diameter and the local irradiation was administered to 8-mm diameter tumor. Effect of the combined treatment was barely evident with the MCa but strongly present in FSa; up to 60% of mice were cured of FSa by C parvum alone and the response to low radiation dose, e.g., 200 rads, was highly increased. For SCC-2 the TCD50 was approximately 7000 rads and 3000 rads in control and test mice, respectively. Comparable values for SCC-4 were 7700 and approximately 5500 rads. Importantly, for SCC-4 there was a large and highly significant reduction in the proportion of mice that died of metastases to lung but were free of evident tumor at the primary site.