We studied MHC class II gene expression and its regulation by IFN-gamma in purified rat peritoneal connective tissue-type mast cells. Mast cells were cultured with or without recombinant rat IFN-gamma (70 ng/ml) for 48 hr and analyzed by RT-PCR for expression of mRNA encoding MHC class II and by fluorescence flow cytometry for surface expression of MHC class II protein product. Levels of MHC class II mRNA and cell-surface protein product in untreated mast cells remained constant throughout the culture period but increased progressively after treatment with IFN-gamma such that by 48 hr levels were significantly greater than those in untreated cells. Dual labeling confirmed that MHC class II product was coexpressed with IgE (a mast cell marker). To conclude, rat connective tissue-type mast cells express mRNA and surface product for the MHC class II gene which can be up-regulated by IFN-gamma.