The currently available antitumoral therapeutic modalities are most often inefficient against metastatic disease. The metastatic phenotype has been shown to be largely determined by cell membrane properties. The cell membrane could therefore be considered as a possible target for antimetastatic drugs. In the present study we examined the effect of hyperthermia (the antitumoral effect of which is based, at least partly, on an action on the cell membrane) on the F1 and F10 variants of B16 melanoma. Cells of the more malignant variant, F10, were found to be markedly more sensitive to hyperthermic treatment than those of the less malignant one, F1. One hour in-vitro treatment by supranormal temperatures (ranging from 40 to 46 degrees C) resulted in a differential effect with regard to both proliferating capacity of the cells in vitro and tumorigenic ability following inoculation to mice. Our present results in the B16 melanoma corroborate data obtained by us in another tumour system, the AKR lymphoma. Study of the effect of membrane-acting agents on tumour variants differing in degree of malignancy might result in the finding of antitumoral agents efficient against advanced cancer.