Further studies were carried out on the combined effects of Corynebacterium parvum and hyperthermia on animal tissues and cultured Chinese hamster ovary cells. Experimental animals were C3Hf/Sed mice derived from our defined flora mouse colony. Tumors were eighth-generation isotransplants of a spontaneous fibrosarcoma, FSa-II. Hyperthermia was given by immersing the mouse foot or culture flasks in the constant temperature water bath. Present experiments include thermal enhancement of C. parvum at different temperatures, effect of the agent on the kinetics of thermal resistance, and the mechanism of the thermal enhancement. The thermal enhancement by C. parvum was independent of temperature in a range between 42.5 and 46.5 degrees, and it increased with decreasing temperature. The analysis of the Arrhenius plot suggested a comparable activation energy for combined treatments and for heat alone between 42.5 and 46.5 degrees. The thermal resistance developed very rapidly in both normal and tumor tissues. Systemic administration of C. parvum failed to modify the kinetics of thermal resistance. Several experiments were attempted in order to disclose the mechanism. A single injection of C. parvum-induced macrophages failed to enhance thermal response of the mouse foot, while 3 daily injections of the macrophages enhanced the response, indicating that the enhancement by C. parvum is at least partly attributed to the C. parvum-induced macrophages. Whole-body irradiation of 6 Gy and/or administration of anti-mouse T-cell serum and histamine failed to inhibit the C. parvum enhancement of thermal response. No thermal enhancement was observed for Chinese hamster ovary cells treated at 43.0 degrees in vitro with C. parvum or thiomersalate , a preservative supplemented in C. parvum, although cytotoxic effect was shown at a high concentration of thiomersalate .