Abstract IIB-MEL-J is a highly heterogeneous newly established human melanoma cell line. The addition of 3 m M, l-tyrosine to the culture medium produced (1) a great decrease in the cell growth rate, (2) a loss of the anchorage-independent growth capacity, and (3) a change in the morphology of the cells to a fibroblastoid aspect. Coincident with these changes, an increase in subpopulations I and II and a decrease in subpopulations III and IV took place. In view of this evidence we consider that the cells have differentiated. The melanin production was not increased by the l-tyrosine treatment, suggesting that differentiation and melanin expression are not strictly correlated.