It was previously reported that treatment with leflunomide (LF; 10 mg/kg/day) together with cyclosporine (CsA; 10 mg/kg/day) resulted in long term survival of hamster heart xenografts (Xg) in rats and that LF could be withdrawn 2 to 4 wk after transplantation. To study the mechanisms allowing withdrawal of LF, second hamster heart Xgs were transplanted 6 wk after the first xenograft. Only the rats that received LF for 4 wk accepted second Xgs (> 30 days; n= 5). Hence, after 4 wk of LF, the rats developed partial B cell tolerance, as they were unable to produce T-independent (CsA-resistant) XAbs. Rejection of second Xgs (2–4 days; n= 5) in the 2-wk LF group resulted in the formation of IgM xenoantibodies (XAbs) localizing together with complement within rejected grafts. However, these XAbs did not affect first Xgs, suggesting that the latter Xgs became resistant to this IgM XAb-mediated rejection, a phenomenon referred to as accommodation. Accommodation was further confirmed as adoptive transfer of IgM XAbs, which resulted in hyperacute Xg rejection in naive rats (< 1 h; n= 5), did not cause rejection in long term survivors (> 30 days; n= 4). This was associated with a down-regulation of the expression on the graft endothelial cells of adhesion molecules (believed to be important expressers of xenogeneic epitopes), such as P-and E-selectins. Interestingly, these adhesion molecules reappeared after retransplanting the accommodated Xgs to naive recipients. In conclusion, depending on the duration of the LF treatment, long term survival of hamster hearts in CsA-treated rats is based in part on accommodation and in part on T-independent B cell tolerance.