The capacity of different cytokines to upregulate major histocompatibility complex (MHC) expression on murine tumor cells in vitro, and on s.c. tumors or pulmonary metastases in vivo has been examined. Interleukins-1, -2, and -4 (IL-1, -2, -4), tumor necrosis factor-α (TNF-α), α-interferon (IFN-α), and γ-interferon (IFN-γ), were incubated with tissue culture lines of murine tumor cells displaying low (MCA-101), intermediate (MCA-102, -106), or high (MCA-105) Class I expression. IFN-α and IFN-γ significantly increased Class I but not Class II antigens on all lines. TNF-α, IL-1, -2, and -4 had no significant effect on Class I or II expression in vitro. Mice bearing pulmonary metastases or s.c. lesions generated by MCA-101 and -102 were treated with IFN-α or IFN-γ i.p. or i.v., or with a single dose of TNF-α i.v. Immunoperoxidase staining of lung metastases or subcutaneous tumors showed an increase in Class I but not Class II expression on MCA-102 tumors treated with IFN-α or IFN-γ. IL-1, -2, or TNF-α had no effect on MHC Class I or II expression in vivo. None of the cytokines tested could upregulate MHC Class I or II expression on MCA-101 tumors in vivo. Flow cytometry analysis demonstrated an increase in Class I but not Class II expression on MCA-102 and MCA-106 tumor cells from s.c. tumor treated with IFN-α or IFN-γ. A kinetic analysis of the flow cytometry data revealed that augmented MCA-102 Class I levels persisted for several days after cessation of in vivo therapy with IFN-α. Our data suggest one possible mechanism for the synergistic antitumor effects of IL-2 and IFN-α.