Hyperthermia-induced antitumor activity was assessed histopathologically and findings related to transplantation of human gastric cancer into nude mice were examined. Fragments of human gastric cancer were incubated at 37 degrees C to 47 degrees C for various durations of time, then were evaluated either histologically or with regard to delay in tumor growth and the rates of transplantation into nude mice. Fragments exposed to 39 degrees C for 30 min to 120 min and 41 degrees C for 30 min did not differ from findings in the controls concerning tumor growth and transplantability. In the case of 41 degrees C for 60 min or more, the rates of transplantation decreased significantly and there was a delay in tumor growth. At 43 degrees C for 120 min, 45 degrees C for 30 and 60 min, and 47 degrees C for 15 to 60 min, transplantability was nil. In the groups with a delay in tumor growth, there were irreversible changes in nucleic and cytoplasmic components, and in 6 groups with no evidence of transplantability, complete destruction of the glandular structure, pyknosis and karyorrhexis as well as disappearance of the cytoplasm occurred in almost all cases. Thus, the thermal dose of 43 degrees C for 120 min seems to be the minimally effective one for hyperthermia-induced tumoricidal activity.