Effect of alpha-melanocyte stimulating hormone (MSH) on proliferating activity of MS and BRO melanoma cell strains with dissimilar phenotype was studied. MSH, apart from the effect on melanomagenesis, influenced the cell proliferation. Growth of MS cells was inhibited at 10(-8)-10(-9) M concentrations of the hormone (IC50 = 2.10(-8) M), while BRO cells were activated. The higher concentrations of MSH inhibited also BRO cells growth (IC50 = 9.10(-7) M). Ligand-receptor interaction of 125I = MSH with plasmatic membranes and cytosol fraction of the cells was studied at 4 degrees and 37 degrees.