In search of a phenotypic marker determining genetically controlled susceptibility to delayed-type hypersensitivity (DTH) reactions in the brain--in particular, experimental autoimmune encephalomyelitis (EAE)--we have compared the gamma-interferon (IFN-gamma) induction of Ia molecules on astrocytes and macrophages from rat and mouse strains that are susceptible or resistant to this disease. We focused on Ia expression because DTH reactions to self or foreign antigens are largely mediated by lymphocytes restricted by class II (Ia) antigens of the major histocompatibility complex (MHC). Our data demonstrate that Lewis (fully susceptible) and Brown Norway (BN) (fully resistant) rats are very different in that Lewis astrocytes express much higher levels of Ia than BN astrocytes. Similar data were obtained from an analysis of EAE-susceptible and -resistant mouse strains (SJL and BALB/c, respectively), which suggests that this phenomenon may be universal and not limited to only one mammalian species. At least one gene responsible for Ia hyperinduction is located outside the rat RT-1 or the mouse MHC locus. Animals congenic at the RT-1 or MHC locus of the resistant strain but with background genes of the susceptible strain exhibit intermediate levels of Ia compared to fully resistant and susceptible rodents, which fits well with the reduced EAE susceptibility of these congenic animals. Furthermore, hyperinduction of Ia is astrocyte specific, since peritoneal macrophages of susceptible and resistant strains exhibit identical profiles of Ia induction. Thus, astrocyte Ia hyperinducibility may be a major strain- and tissue-specific factor that contributes to Ia-restricted DTH reactions in the brain.