Isolated human and mouse pancreatic islet cells and the rat insulinoma cell line RIN-m5F were used to examine the ability of recombinant interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) to regulate the expression of the class I and class II major histocompatibility (MHC) surface proteins and mRNA in β-cells. Each cytokine increased significantly the expression of class I MHC proteins as determined by double indirect immunofluorescence microscopy and flow cytofluorimetric analysis. In the RIN-m5F cells, this increase in surface expressed class I MHC proteins was mirrored by an increase in the level of class I MHC mRNA. The order of potency of the cytokines on class I MHC expression was TNF-α plus IFN_-γ≥ IFN_-γ≥ TNF_-α. While IFN_-γ or TNF-α alone were without effect, in combination they were found to induce class II MHC proteins on 30–40% of human or murine β-cells. In contrast, IFN_-γ plus TNFα did not induce detectable class II MHC proteins or mRNA in the RIN-m5F cells. These findings indicate that 1) TNF-α, in addition to IFN_-γ, upregulates the expression of β-cell class I MHC proteins and mRNA, and 2) more than one signal is required for the induction of class II MHC proteins on β-cells. The ability of IFN_-γ plus TNF_-α to induce class II MHC proteins on only a fraction of the normal β-cell population and not on RIN-m5F cells suggests that this response is related to the differentiation state of the β-cell.