Genetic association of some immune-mediated human uveitic diseases with histocompatibility antigens, ethnic origin, familial background, or gender have suggested the presence a hereditary component in susceptibility. Experimental autoimmune uveoretinitis (EAU) can be induced in inbred rodents by immunization with evolutionarily conserved retinal proteins, and mimics many features of human uveitis. Susceptibility to EAU is genetically controlled, and the model is being used to study mechanisms that might affect susceptibility to ocular autoimmune disease. EAU expression in mice and in rats requires the presence of both a susceptible MHC haplotype and a "permissive" genetic background. MHC control of susceptibility in H-2k mice was tentatively mapped to the I-A subregion (HLA-DR equivalent), implicating epitope recognition as a major mechanism in susceptibility. In contrast, expression of the I-Ek gene product (HLA-DQ equivalent) appeared to have an ameliorating effect on disease. Susceptible H-2 haplotypes exhibited highest disease scores on the B10 background, and disease was reduced, or even absent, on some other (nonpermissive) backgrounds. Factors which may determine "permissiveness" or "nonpermissiveness" of a particular genetic background, as studied in mice and rats, may include regulation of responses to lymphokines, hypothalamic-adrenal-pituitary axis hormones, mast cell/vascular effects, and possibly the T cell repertoire. The data are interpreted to suggest that, in individuals susceptible to uveitis by virtue of their MHC, the final expression of disease will be determined by the genetic background. These results might help to explain why only a minority of individuals with a susceptible HLA type develop uveitis, as well as the variable incidence of disease in HLA-identical populations of different ethnic backgrounds.