Genistein, an in vitro inhibitor of topoisomerase II and tyrosine kinases, elicited an inhibition of growth and increased melanin content in five human melanoma cell lines, after six days of treatment at a concentration of 45 μM. In two lines examined more thoroughly, HO and SK-MEL-131, treatment with genistein also increased other markers of differentiation, including tyrosinase activity, reactivity with CF21 monoclonal antibody, and dendrite-like structure formation. The genistein-evoked increases in melanin content and tyrosinase activity were concentration- and time-dependent. Treatment of HO and SK-MEL-131 cells with 45 μM genistein for 24 hr or 60–600 μM genistein for only 1 hr resulted in an increase in protein-linked DNA strand breaks. Our results suggest an association between the genistein-evoked, protein-linked, DNA strand breaks and the genistein-induced differentiation of human melanoma cells.