Melanocytes, highly differentiated neural crest-derived cells, are located in the basal layer of the epidermis, where they play a role in protecting against UV damage in the skin. Previous studies suggest that both growth hormone (GH) and the insulin-like growth factor I (GH/IGF-I) system may be important for melanocyte growth and function. We have therefore characterized the role of the GH/IGF system in melanocyte growth in vitro and its interaction with the local growth factor basic fibroblast growth factor (bFGF). Analysis of the effects of GH, IGF-I, and bFGF and combinations of these growth factors on melanocyte growth in vitro revealed that 1) GH stimulates the growth of melanocytes when combined with IGF-I, des(1-3)IGF-I [an analog of IGF-I that has a reduced binding affinity for IGF-binding proteins (IGFBPs)], or bFGF, either separately or in combination; 2) in contrast to the lack of effect of GH or bFGF alone, both IGF-I and des(1-3)IGF-I enhance melanocyte growth in a dose-dependent manner; and 3) IGF-I is more efficacious in eliciting a growth response at low concentrations compared to des(1-3)IGF-I. Using Western ligand blotting, affinity cross-linking, immunoprecipitation, RIA, and Northern analysis, we show that cultured human melanocytes synthesize and secrete minimal amounts of IGFBP. IGFBP-4 is the major IGFBP produced by these cells when cultured in complete growth medium or in the presence of either IGF-I or des(1-3)IGF-I alone. In conclusion, these studies provide support for a role for both GH and IGF-I in the growth of human melanocytes in vitro, involving synergy with bFGF. Low levels of melanocyte-derived IGFBP-4 may play a role in enhancing the modulation of IGF action.