Sera obtained from kidney transplant recipients at various times after the graft were investigated for the presence of cytotoxic antibodies to B and T human lymphoid cells and for their ability to inhibit mixed lymphocyte reaction between allogeneic lymphocytes. More than 50% of sera from 35 such patients contained cytotoxic antibodies to B lymphocytes and 27 of 33 sera blocked mixed lymphocyte reaction by 25 to 75%. A statistically significant association between the presence of cytotoxic antibodies to B cells and the ability to block the mixed lymphocyte reaction was found in the group of patients with functioning grafts. However, the presence of cytotoxic antibodies to B lymphocytes did not correlate with the outcome of the graft.