Astrocytes, astrocytic cell lines and endothelium from BDIX rats were stimulated with recombinant interferon-gamma (IFN-gamma) and the expression of MHC molecules quantified using an enzyme immunoassay (EIA). Using the two mouse anti-RT1.B monoclonal antibodies MRC OX4 and OX6, previously described as recognizing a monomorphic determinant on RT1.B, as well as polyvalent rabbit anti-rat class II antisera, we were unable to demonstrate any induction of RT1.B molecules on these cells under conditions that induced RT1.B expression in all other strains tested. In contrast, RT1.D locus class II molecules, detectable by the antibody MRC OX17, are more strongly expressed in BDIX than in other strains. In experiments using BDIX lymphocytes, this serologically detected defect in RT.1B expression was confirmed using four additional mouse anti-mouse I-Ak monoclonal antibodies, which cross-reacted on all rat strains tested except BDIX. It appears likely that BDIX rats lack either a structural or controlling gene required for RT1.B expression.