The effects of heat and the interaction between hyperthermia and alkylating agents, such as cisplatin (CDDP) and melphalan (L-PAM) in human malignant melanoma biopsies have been investigated by a short-term assay based upon the inhibition of 3H-thymidine incorporation. Cell suspensions from 50 cutaneous and lymph nodal metastases were heated at 40.5 degrees C or at 42 degrees C for 1 h. There were significant antiproliferative effects due to heat in 10% of the tumors exposed to 40.5 degrees C and 34% to 42 degrees C. Thermal resistance was evident in 73% (at 40.5 degrees C) and 54% (at 43 degrees C) of tumors, and there was significant enhancement of cell growth in 17% and 12% of tumors. The combined effects of hyperthermia and drugs were studied on 36 tumors. Cell suspensions were exposed to different concentrations of CDDP or L-PAM for 1 h at 40.5 degrees C and 42 degrees C. Synergy between heat and CDDP was observed in 7% of cases treated with the lowest drug dose and 38% of cases treated with the highest (40.5 degrees C), with only a slight increase in the frequency of synergy at 42 degrees C. Synergy between heat and L-PAM was also observed in 12% to 44% of tumors at 42 degrees C as a function of drug concentration.