It has been shown that all three classes of interferons enhance the expression of the major histocompatibility class I antigens (HLA-A,B,C;H-2) on a wide variety of cell types (1-10). However, their effect on the expression of the class II antigens (HLA-DR, Ia), which play a major part in cellular interactions that initiate an immune response, is more controversial. The predominate findings have been that the interferons specifically increase the synthesis and expression of only the class I antigens (3, 4, 6, 8, 10, 11). We report here that recombinant interferon-gamma (IFN-gamma) increases the synthesis and expression of the HLA-DR (la-like) antigens as well as beta 2-microglobulin (beta 2-m), a low m.w. subunit of HLA, on human melanoma cells. No increase in HLA-DR was detected on these melanoma cells with leukocyte interferon (IFN-alpha) at doses 400 times higher than the maximum dose of IFN-gamma. These findings were extended to show that pure IFN-gamma also increases the expression of the HLA-DR antigens on normal peripheral blood monocytes, whereas recombinant IFN-alpha at a similar dose had little effect on the expression of this surface antigen. These findings suggest a specialized role for IFN-gamma in immune regulation in comparison with IFN-alpha.