Normal melanocytes require a number of exogenous growth factors in contrast to most metastatic malignant melanomas. This investigation demonstrates that endogenously produced human IL-8 can act as an important growth factor for human melanoma cells. In the present study, six out of eight human melanoma cell lines tested secrete IL-8 protein into the culture supernatant. In two of these IL-8-secreting melanoma cell lines, SK-MEL 13 and SK-MEL 23, we have determined the IL-8 requirement for their proliferative capacity. These melanoma cell lines produced significant amounts of bioactive IL-8 as measured by the ELISA technique. Secretion of human IL-8 was inducible by IL-1 and by PMA. Human IL-8-specific mRNA was already detected in unstimulated melanoma cells. In addition, human IL-8-R mRNA could be detected for the first time in human melanoma cells. Exposure of the two melanoma cell lines in vitro to antisense oligonucleotides targeted against two different sites of human IL-8 mRNA-inhibited cell proliferation, colony formation in soft agar, and secretion of IL-8 protein into culture supernatant in a dose dependent fashion. Effects were reversible either by removal of the oligomers or by addition of exogenous IL-8 protein. In contrast, exposure to IL-8 sense probes or oligonucleotides in sense or antisense orientation specific for IL-7, TGF-alpha, TGF-beta, and MGSA had no such effect. A monospecific immune serum and two IL-8-specific mAb were also capable of inhibiting melanoma cell proliferation in the same manner. These results provide strong evidence for an autocrine IL-8 synthesis and for an IL-8-dependent proliferation in a subgroup of human melanomas. Furthermore, they suggest that IL-8 may play a role not only in immunomodulation but also in melanoma progression and metastatic spread.