The autosomal recessive lpr and gld genes induce in mice multiple autoantibodies and the progressive accumulation of large numbers of non-malignant CD4^- CD8^- T lymphocytes. The clinical syndromes and immune abnormalities associated with these two nonallelic genes are nearly identical and are also highly dependent on background genes. MRL/lpr mice are particularly severely affected, and they develop a syndrome that is serologically and pathologically similar to human systemic lupus erythematosus (SLE). Abnormal cell marker expression in the aberrant lpr T lymphocytes includes surface antigens normally associated with activated T cells or even with B cells, and it occurs along with enhanced expression of certain oncogenes. The lpr gene results in intrinsic abnormalities of both T and B lymphocytes, yet its location and product are unknown. The gld gene is located on chromosome 1; its product is also unknown. Although many immunological abnormalities are known, the mechanism whereby these two genes induce autoimmunity and lymphoproliferation remains obscure. Further studies of mice bearing these mutant genes are certain to yield insights into systemic autoimmunity and the control of lymphocyte proliferation.