The response of class I major histocompatibility complex antigen expression to in vitro administration of interferon and tumor necrosis factor alpha (TNF-alpha) was measured using class I major histocompatibility complex-deficient small cell lung cancer cell lines. Significant induction also was observed using gamma interferon (IFN-gamma) alone, whereas TNF-alpha alone yielded only modest induction. Classic small cell lung cancer cell lines NCI-H146 and NCI-H209 best demonstrated synergistic HLA and beta 2-microglobulin antigen induction with IFN-gamma and TNF-alpha with the following dose schedule: 3-6 days of TNF-alpha (200 units/ml) followed by 48 h of IFN-gamma (100 IU/ml). Induction was quantitated using an 125I-Protein A radioimmunoassay. Synergistic induction of the HLA and beta 2-microglobulin surface antigens on NCI-H146 was also possible with alpha interferon and TNF-alpha but required a higher concentration of the interferon, i.e., 3-6 days of TNF-alpha (200 units/ml) followed by 48 h of alpha interferon (1000 units/ml). Small cell lung cancer cell line NCI-H146 was further studied for expression of major histocompatibility complex messenger RNA using the optimal doses and sequence of addition of IFN-gamma and TNF-alpha as indicated above. A significant induction with IFN-gamma alone and synergistic induction with both IFN-gamma and TNF-alpha was quantitated for both HLA-A2 and beta 2-microglobulin transcripts using Northern blot analysis. Incubation with relatively low subcytotoxic doses of IFN-gamma and TNF-alpha also resulted in a marked synergistic decrease in c-myc message.