The modulating effects of hyperthermia on cytotoxity and oncogenicity of several chemotherapeutic agents were investigated using the C3H 10T1/2 cell system. Logarithmic phase cultures of 10T1/2 cells were treated with various doses of cis-platinum or bleomycin sulphate for 2 h, either alone or with simultaneous hyperthermia (42 degrees C for 2 h). In a second set of experiments, cells were treated for 24 h with either melphalan or cis-platinum followed by a 2 h heat treatment. While hyperthermia alone was found to be ineffective in inducing oncogenic transformation and was only moderately cytotoxic, concurrent hyperthermia with drug treatment enhanced both the toxicity and oncogenic transforming potential of drugs 3-4-fold in C3H 10T1/2 cells. Sequential heat after drug treatment, however, was found to increase toxicity slightly but had no effect on the oncogenicity of the drugs.