Rejected MHC-mismatched cardiac allografts in CCR5−/− recipients have low T cell infiltration, but intense deposition of C3d in the large vessels and capillaries of the graft, characteristics of Ab-mediated rejection. The roles of donor-specific Ab and CD4 and CD8 T cell responses in the rejection of complete MHC-mismatched heart grafts by CCR5−/− recipients were directly investigated. Wild-type C57BL/6 and B6. CCR5−/−(H-2 b) recipients of A/J (H-2 a) cardiac allografts had equivalent numbers of donor-reactive CD4 T cells producing IFN-γ, whereas CD4 T cells producing IL-4 were increased in CCR5−/− recipients. Numbers of donor-reactive CD8 T cells producing IFN-γ were reduced 60% in CCR5−/− recipients. Day 8 posttransplant serum titers of donor-specific Ab were 15-to 25-fold higher in CCR5−/− allograft recipients, and transfer of this serum provoked cardiac allograft rejection in RAG-1−/− recipients within 14 days, whereas transfer of either serum from wild-type recipients or immune serum from CCR5-deficient recipients diluted to titers observed in wild-type recipients did not mediate this rejection. Wild-type C57BL/6 and B6. CCR5−/− recipients rejected A/J cardiac grafts by day 11, whereas rejection was delayed (day 12–60, mean 21 days) in μMT−/−/CCR5−/− recipients. These results indicate that the donor-specific Ab produced in CCR5−/− heart allograft recipients is sufficient to directly mediate graft rejection, and the absence of recipient CCR5 expression has differential effects on the priming of alloreactive CD4 and CD8 T cells.