The expression of major histocompatibility complex (MHC) antigens (class I and II), type 3 complement receptor (CR3) and leucocyte common antigen (LCA) was examined in epiplexus cells in rats of different ages. The cells exhibited intense immunoreactivity with the monoclonal antibody OX-42 which recognizes CR3 receptors. In early postnatal rats (1 d), the immunolabelled cells were mostly round but with increasing age (7 wk), they assumed a ramified or elongated form. The expression of LCA marked by the monoclonal antibody OX-1 followed a similar staining pattern. Class I MHC antigen expression was also demonstrated in some epiplexus cells using the monoclonal antibody OX-18 but they were less numerous than the OX-42 or OX-1 positive cells. Only sporadic OX-6 positive cells were observed in postnatal rats but they showed a marked increase in number in adult rats, suggesting an upregulation of class II MHC antigens with age. The expression of MHC class II antigens was vigorously elevated in postnatal rats receiving 6 successive intraperitoneal (i.p.) injections of interferon gamma (IFN-gamma). In these animals, a large number of intensely stained OX-6 positive epiplexus cells were observed. These were mostly elongated or ramified with long processes. The immunostaining of epiplexus cells with OX-18 was also enhanced after IFN-gamma injections but the expression of CR3 and LCA appeared to be unaffected. It is concluded that the expression of MHC class I and II antigens on epiplexus cells is upregulated and induced respectively after successive i.p. injections of IFN-gamma into postnatal rats.(ABSTRACT TRUNCATED AT 250 WORDS)