Anti-HLA-A,B and anti-B lymphocyte antibodies were screened as part of a prospective alloimmunity monitoring study in 29 renal allograft recipients using a standard microlymphocytotoxicity test. Warm-reactive and/or cold-reactive lymphocytotoxins were directed against a panel of B lymphocytes, the donor's B lymphocytes, and the recipient's own B lymphocytes. A small proportion of patients had pretransplant antibodies, whereas about one-half of the patients had post-transplant antibodies. One-year allograft survival rates were lower among the patients with warm- and cold-reactive sera than among those with nonreactive sera or pure B cold-reactive sera. The sera of 20 patients were tested against donor B lymphocytes. The presence of donor-specific antibodies correlated closely enough with graft loss to be of predictive value. Autoantibodies appeared to have an enhancing effect in this study.