Normal human cells, cells from nonmalignant proliferative lesions, and primary and metastatic tumor cells can be maintained in vitro and analyzed for requirements for growth in chemically defined media. The human melanocytic cell system with normal melanocytes, precursor nevus cells, and primary and metastatic melanoma cells has been extensively studied for the phenotypic properties of the cells, including their requirements for exogenous growth factors and other mitogens. In high calcium-containing W489 medium, normal melanocytes require four supplements: IGF-I (or insulin); bFGF, TPA, and alpha-MSH. Nevus cells are largely independent of bFGF. Depletion of TPA from medium is not as detrimental to nevus cells as it is to melanocytes, but the phorbol ester is still essential for maintenance of the typical nevic phenotype. Primary melanoma cells require at least one growth factor, IGF-I (or insulin), for continuous proliferation. On the other hand, metastatic cells of melanoma as well as of carcinomas of colon and rectum, bladder, ovary, and cervix are able to proliferate after a short adaptation period in medium depleted of any growth factors and other proteins. Doubling times of metastatic tumor cells in protein-free medium are only 30-60% longer than in FCS-containing medium. The growth autonomy of human tumor cells is apparently due to the endogenous production of growth factors. Likely candidates for autocrine growth stimulation of human tumor cells are TGF-alpha, TGF-beta, and PDGF. Melanoma and colorectal carcinoma cells express functional EGF/TGF-alpha receptors, and produce TGF-alpha, indicating that this growth factor is produced for autocrine stimulation. In addition to the use of anti-growth factor antibodies, other strategies for the inhibition of autocrine growth stimulation include mAbs to growth factor receptors, soluble receptors, receptor-mimicking antiidiotype antibodies, and active immunization against growth factors. Whether any of these therapeutic approaches is clinically feasible will need to be determined in extensive preclinical investigations.