In B16 melanoma, cells morphologically different can be distinguished. In order to establish possible correlations between cell morphology and drug-response, the cytotoxic response to doxorubicin was analyzed. The two subpopulations, represented by two types of colonies, showed a different degree of sensitivity to doxorubicin. Moreover, following treatment, colonies strongly altered in their morphology were found, suggesting a differentiating activity of doxorubicin (dendritic prolongations, increase of intracellular melanin, block of cell proliferation). These results suggest that doxorubicin, besides having a different cytotoxic effect on the two cell subpopulations, induces in this cell line morphological alterations consistent with a differentiation process.