Rat renal allograft survival may be enhanced by active preimmunization, or by passive transfer of antidonor antibodies. In the (LEW X BN)F1 to LEW X LEW model, antibodies to Ia-like (B region) antigens, but not Ag-B (A region) antigens promote passive enhancement. Serum antibody responses to Ia and Ag-B antigens were assessed serially in passively enhanced animals to see whether there were similar quantitative and qualitative suppressions of the immune response, as compared to unmodified rejecting controls. Acid eluates from homogenized allografts were also studied and compared to serum activities. Rejecting animals produced antibodies to both Ag-B and la gene products, rising sharply between days 4 and 7 after transplantation. Passively enhanced recipients made no detectable anti-Ia, and very weak anti-Ag-B, responses. Active immunization with cells produced anti-Ag-B and anti-Ia responses 1 week later; after transplantation there was disappearance of serum anti-Ag-B and some reduction in anti-Ia levels. When graft eluates were prepared 7 days after transplantation, passively enhanced kidneys had only traces of antibody activity, while actively enhanced kidneys had anti-Ag-B, but no anti-Ia, compared to controls in which both types of antibodies were readily recovered. Hence, antibody responses to gene products of different regions of the rat major histocompatibility complex (MHC) are not identical in states of rejection, passive, or active enhancement.