The Y-linked autoimmune accelerating gene mutation (yaa), first discovered in the BXSB mouse strain, is known to accelerate spontaneous autoantibody production and subsequent development of lupus disease. We have investigated the role of the yaa gene in the development of the type I1 collagen (CI1)-induced arthritis (CIA), which is used as a model for rheumatoid arthritis. In contrast to the accelerating effects on development of lupus autoimmunity we can show that the presence of BXSB Y chromosome carrying the yaa gene block development of CIA in F1 crosses with three normally CIA-susceptible strains, DBA/l, C3H.Q and Bl0.Q. Backcross experiments showed an additional modulatory effect from other BXSB genes or possibly from DBA/1 X chromosome.To evaluate the effect mediated by the yaa gene alone, the BXSB Y chromosome was bred into the DBA/1 gene background. The DBA/1 congenic DBA/1 .yaa male mice were less susceptible to arthritis development than their DBA/l counterparts. (BlO.QxDBA/l .yaa)FI acquired resistance to arthritis development similar to that of DBA/l.yaa, indicating a role for the yaa gene alone.The serum levels of autoantibodies to CII were significantly suppressed in all strains carrying yaa. In DBA/l.yaa mice a reduced number of T cells were found to produce interferon-y after in vitro stimulation with CII. Thus, although autoreactive B cells are important in both diseases they play different roles in murine lupus and in CIA.