Two recently described murine strains, MRL/1 and BXSB, develop a lupus-like syndrome resulting in a 50% mortality by the fifth month of age. Comparison of the immunopathological and virological characteristics of these mice with those of the NZB/NZW F1 mouse reveals several pathogenetic common denominators but no obvious common etiologic factors. In all three kinds of mice, the lupus-like syndrome consists of a fatal immune complex type glomerulonephritis and complete or near complete thymic cortical atrophy plus lymphoid hyperplasia that varies in degree among the three kinds of mice. The nephritic glomeruli contain a concentration of antinuclear antibodies plus varying amounts of stainable gp70. This syndrome is consistently correlated with abnormally elevated serum IgG levels, antinuclear antibodies, anti ds- and ssDNA antibodies, and circulating immune complexes, as well as depressed serum hemolytic complement. Features that differ among the three kinds of mice include: H2 type, anti-lymphocyte antibody, cryoglobulins, T-B cell ratios, sex incidence of disease, vasculitis, and oncornaviral flora. The serum gp70 levels in the three mice also differ considerably, but all are within the range of gp70 levels found in some immunologically normal strains.