The antitumor activities of human interferon (IFN) alpha, beta, and gamma alone or in combination were studied on four human melanoma cell lines (StML-11, StML-12, StML-14, and SKMel-28) in various concentrations (1-50,000 IU/ml IFN alpha, 0.1-1000 IU/ml IFN beta, 1-10,000 IU/ml IFN gamma) in vitro. In all experiments IFN beta exhibited the most potent antiproliferative effect of all IFN tested. After 3 d of incubation a 50% growth inhibition was achieved with 20-40 IU/ml for natural IFN beta and with 600-1200 U/ml for recombinant IFN gamma. Substantially higher doses (7,000 to more than 50,000 IU/ml) of recombinant IFN alpha 2a were required to achieve a 50% growth inhibition. A strong synergistic antiproliferative activity resulted from the combination of IFN alpha with IFN gamma and IFN beta with IFN gamma. None of the IFN tested induced terminal differentiation of melanoma cells in vitro. The formation of dendrites was inhibited, and the portion of differentiated cells in vitro was reduced after treatment with IFN in comparison to the untreated controls (untreated controls: 100%; portion of differentiated cells after treatment with IFN alpha: 58%-74%, IFN beta: 48%-96%, IFN gamma: 10%-33%). The melanin synthesis was slightly elevated after treatment with IFN alpha (untreated controls: 100%; after treatment with IFN alpha: 103%-157%, ns.) and decreased significantly after treatment with IFN beta (49%-71%, p less than 0.05) as well as with IFN gamma (80%-88%, ns.). Cell surface markers were modulated varyingly by the IFN: HLA-I antigens were enhanced by all IFN, with IFN beta emerging as the most potent inducer. Only IFN gamma, however, induced a de novo expression of HLA-DR and -DQ antigens and increased the expression of the ICAM-1 molecule and of the melanoma progression marker A.1.43. Possibly, these findings indicate a biologically more aggressive phenotype of melanoma cells.