BXSB male mice spontaneously develop progressive autoimmune disease characterized by high serum immunoglobulins, including anti-nuclear antibodies (ANA), enlarged spleen and lymph nodes, and diffuse proliferative glomerulonephritis. Females develop symptoms at a much slower rate. The mechanisms underlying the autoimmune disease and the nature of the Y-linked accelerating factor have not yet been elucidated. We found that the male progeny of the cross between the non-autoimmune strain CBA/Ca and BXSB (CBA/Ca X BXSB)F1 showed progressing signs of autoimmunity starting at 6 to 7 mo. In contrast, the male progeny that resulted from BXSB males crossed with immune-defective CBA/N females (Xid) were devoid of splenic B colonies, were nonresponsive to TNP-Ficoll, and were free of autoimmune disease for at least 10 mo. At 18 mo, some of the (CBA/N X BXSB)F1 mice developed weak antinuclear antibodies, but no spleen or lymph node enlargement was seen. The same mice had low anti-TNP Ficoll responses but did not produce B colonies in vitro. The role of the X chromosome in regulating expression of autoimmunity in young and old BXSB mice is discussed.