Normal human melanocytes in culture require specific additives such as basic fibroblast growth factor (bFGF) and dibutyryl cyclic adenosine monophosphate (dbcAMP) in order to proliferate in defined or serum-containing medium (Halaban et al., 1987). This stringent requirement is absent in cells derived from metastatic melanomas which not only proliferate in regular culture medium, but also produce a substance immunologically related to bFGF (Halaban et al., 1987). We show here that the mitogenic activity necessary for normal human melanocytes is constitutively present in several lines of human metastatic melanomas and that this activity is inactivated by anti-bFGF antibodies. Melanoma cells, but not normal melanocytes, express bFGF gene transcripts. Although the molecular mechanism underlying the abnormal expression of bFGF in melanomas is not known, the results suggest that bFGF acts as an autocrine growth factor in melanomas.