Allele-specific monoclonal anti-I-A antibodies are capable of specifically suppressing the immune response to antigens under the control of the allele towards which the antibody is directed, without suppressing the response to antigens under the control of the alternative allele of the I-A alpha and beta chain genes in an F1 heterozygote. This phenomenon, which has been termed 'allele-specific immunosuppression', is antigen-specific, long-lasting and transferrable with Thy-1-positive spleen cells. This type of immunosuppression has been applied to animal models of autoimmune disease, in both homozygous and heterozygous animal models. Anti-I-A monoclonal antibodies are capable of preventing, suppressing and treating experimental allergic encephalomyelitis (EAE), of partially suppressing experimental autoimmune myasthenia gravis, and of preventing the onset of type I insulin-dependent diabetes in the BB/W diabetic rat. In addition, this type of immunotherapy has succeeded in almost completely suppressing nephritis in NZB X NZW F1 mice, which normally develop severe lupus-like nephritis. Significant toxicity, which may be due to anti-allotype antibodies, anti-idiotype antibodies, or to impurities in the monoclonal antibody preparations, has been encountered in the BB/W diabetic rat. In addition, attempts to extend these observations to EAE in the cynomolgus monkey have encountered significant mortality which appears to be attributable to the monoclonal antibody injections (anti-HLA-DR). The mechanism of this toxicity and means of circumventing it are currently under investigation. These results demonstrate the critical role of I-A molecules in the induction and continuance of the autoimmune process in these experimental animal models.