We investigated die relation between the thermal sensitivity and the cell kinetics and ploidy of human malignant melanoma primary cultures. Cell suspensions from lymph node and/or cutaneous metastases of 49 patients were exposed for 1 h to 42-degrees-C. The effect of hyperthermia was assessed by an antiproliferative assay based on H-3-thymidine incorporation of cells grown in agarose for 4 days. Thermal sensitivity was observed in 10 of the 49 tumors studied (20%). In vitro thermal response as a function of proliferative activity (H-3-thymidine labeling index) was investigated in 36 melanomas. The median H-3-thymidine labeling index of sensitive tumors was 4-fold (8.1% vs 2.0%) that of resistant tumors. The relation between in vitro thermal sensitivity and flow cytometric DNA content was possible to find in 21 malignant melanomas. Thermosensitivity was found in 2 of the 3 diploid melanomas and in 2 of the 10 tumors with only one aneuploid subpopulation, whereas all multiploid tumors showed in vitro thermosistance. Our findings suggest that cell kinetic characteristics and, ploidy could be considered potential indicators of thermoresponsiveness in human malignant melanoma.