Autoimmune disease in BXSB mice progresses more rapidly in male animals. We have investigated the cellular basis of this effect by transferring male and female bone marrow and spleen cells into male and female lethally irradiated BXSB recipients. The rate of development of disease was measured by the overall mortality rate, mortality from glomerulonephritis, and development of serologic abnormalities. We found that the pace of disease in the BXSB sex chimeras was determined entirely by the sex of the donor of the transferred cells. Lethally irradiated BXSB animals receiving male cells had rapidly progressive disease, whether the recipients were themselves male or female, whereas female-cell recipients of either sex had slowly progressive disease. The male-specific effect that accelerates autoimmune disease in the BXSB is thus not hormonally mediated, but rather is expressed in the hematopoietic stem cell populations.