The proliferative activity of cells of malignant melanomas.

The proliferative rates of cells of malignant melanomas were studied by autoradiography after incorporation of tritiated thymidine in them. There is a relationship between the proportion of melanomatous cells synthesizing DNA and the maximum thickness of the neoplasm. Our data suggest that there are two biologically distinct steps in the evolution of malignant melanoma that are closely related to the direction of growth of the neoplasm (horizontal or vertical) but unrelated to the clinicopathological pattern (superficial spreading, nodular or lentiginous). Malignant melanomas with high risk for early metastases are characterized by higher labeling indices for tritiated thymidine (greater than 1.5%) and by greater numbers of melanomatous cells synthesizing DNA (greater than 40/mm2 at the lateral margins of the neoplasms).