CD4 T cell-dependent mechanisms promoting allograft rejection include expression of inflammatory functions within the graft and the provision of help for donor-reactive CD8 T cell and Ab responses. These studies tested CD4 T cell-mediated rejection of MHC-mismatched cardiac allografts in the absence of both CD8 T and B lymphocytes. Whereas wild-type C57BL/6 recipients depleted of CD8 T cells rejected A/J cardiac grafts within 10 days, allografts were not rejected in B cell-deficient B6. μMT−/− recipients depleted of CD8 T cells. Isolated wild-type C57BL/6 and B6. μMT−/− CD4 T cells had nearly equivalent in vivo alloreactive proliferative responses. CD4 T cell numbers in B6. μMT−/− spleens were 10% of that in wild-type mice but were only slightly decreased in peripheral lymph nodes. CD8 T cell depletion did not abrogate B6. μMT−/− mice rejection of A/J skin allografts and this rejection rendered these recipients able to reject A/J cardiac allografts. Redirection of the alloimmune response to the lymph nodes by splenectomy conferred the ability of B6. μMT−/− CD4 T cells to reject cardiac allografts. These results indicate that the low number of splenic CD4 T cells in B6. μMT−/− mice underlies the inability to reject cardiac allografts and this inability is overcome by diverting the CD4 T cell response to the peripheral lymph nodes.