Although CD4-targeted therapy markedly prolongs survival of organ allografts in naive rodents, its effects in primed hosts have not been studied. In our model of accelerated rejection (ACCR) of cardiac Tx in rats, treatment with BWH-4, a CD4 mAb (IgG2a), in the sensitization (between skin and heart Tx) but not in the effector (after cardiac Tx) phase, abrogated fulminant less than 36 hr rejection response and prolonged Tx survival to ca. 11 days. This effect correlated with decreased frequency of circulating CD4+ cells, but it did not depend upon their total depletion. It was also related to BWH-4 mAb-mediated elimination/depression of strong anti-donor humoral responses and cellular responses as determined by lymphocyte-mediated cytotoxicity and mixed lymphocyte reaction and mounted otherwise at the time of engraftment by untreated sensitized hosts. Immunoperoxidase studies of cardiac Tx from BWH-4-conditioned recipients revealed reduced T and B cell activities, reflected in abolition/reduction in deposition of humoral mediators, infiltrating cells, intra-Tx elaboration of interleukin-2 and interferon-gamma, and cell activation. This first report of the successful use of CD4 mAb in sensitized recipients of vascularized organ Tx, stresses the role of CD4+ cells as potential targets for immunosuppression in the sensitization phase of accelerated Tx injury. The beneficial therapeutic effect, probably due to both depletion and functional inhibition of CD4+ T cells, has been achieved by using relatively low doses of BWH-4 mAb.