A morphometric analysis was carried out on electron micrographs of cells of the F1 (low metastatic capacity) and F10 (high metastatic capacity) variant sublines of the murine B16 melanoma, both in in-vitro cultures and in lung-metastatic nodules developed after the intravenous injection of neoplastic cells in syngeneic C57 black male mice. A group of 28 morphometric parameters was derived to describe quantitatively each neoplastic cell profile. No qualitative difference was observed between the two cell lines. The quantitative expression of subcellular organelles was dissimilar in the two sublines, being consistently characterized, both in in-vitro cultured cells and in lung-metastatic colonies, by a significant decrease in the mean values of parameters related to melanosomes in the high metastatic capacity cell line (B16-F10). Moreover, in in-vitro cultured cells, indices describing heterochromatin masses and cytoplasmic membranous compartments displayed statistically significant differences between the two sublines. In this experimental system, an inverse relationship between metastatic capacity and differentiation is detected, since cells with a more aggressive metastatic behavior exhibit a decreased degree of differentiation.