The survival of four human fibroblast strains and seven malignant melanoma cell lines was determined by the colony formation method following 4-hr treatment with various concentrations of melphalan, with or without simultaneous exposure to hyperthermia (42 degrees). The two amelanotic melanoma lines (MM127 and MM253) were 10 times more sensitive to melphalan at 36 degrees than were the four fibroblast strains, the five pigmented melanoma lines being of intermediate sensitivity. Sensitivity to melphalan was usually accompanied by sensitivity to heat, while combined treatment was not only synergistic in most lines but increased the differential between fibroblasts and melanoma cells. Survival studies carried out at 36 degrees, 40 degrees, 42 degrees, and 44 degrees, using human fetal lung fibroblasts and MM253 cells, showed that 42 degrees gave the greatest differential effect and allowed reasonable survival of the normal cells. Time-survival comparison of the same two lines demonstrated that there was no advantage in prolonging hyperthermia unless melphalan was used. A more convenient method for determination of survival was developed based on thymidine uptake of colonies grown in Linbro wells.