The role of humoral immunity in graft rejection in the rat model remains controversial. Passive transfer of cytotoxic alloantibody (CAA) has resulted either in hyperacute rejection or in graft enhancement. This study examines the effect of transfer of CAA on cardiac allograft survival in three rat strain combinations that are fully mismatched at the major histocompatibility (MHC) loci. Strain-specific immune responsiveness in donorrecipient pairs varied from low (Lewis-to-ACI) to high (ACI-to-Lewis) as measured by mixed lymphocyte reactions. CAA was obtained from rats sensitized by three successive skin grafts at weekly intervals. Group 1 (high responder recipients), which consisted of Lewis rats presensitized to ACI and had a lymphocytotoxicity titer of 1:512 to 1:2048, rejected ACI cardiac allografts in 10.8±7.2 hr compared with 6.5±0.5 days in naive controls (P<0.001). Injection of 1 ml of high-titer CAA into naive Lewis rats immediately after ACI cardiac grafting led to hyperacute rejection of ACI hearts in 2.1±0.8 hr while 1 ml of CAA followed by 2 ml of guinea pig complement (GPC) resulted in even faster rejection (mean survival time (MST) of 23.8±4.7 min). Injection of 2 ml GPC alone or in combination with 1 ml naive Lewis serum had no effect on graft survival. Multiple pretransplant injections of 1 ml of CAA on days —3, —2, —1, and 0 relative to transplantation resulted in significant prolongation of allograft survival (MST of 10.3± 0.3 days;P<0.01). In group 2 (intermediate responder recipients), where Lewis rats were presensitized to WF