The cellular basis of the specific unresponsiveness that develops in DA rats treated with cyclosporine (CSA) for 10 d after grafting a PVG heart was examined using an adoptive transfer assay. CD4+ cells from rats with long survival grafts specifically lack the capacity to restore PVG heart graft rejection, and can also inhibit the capacity of naive T cells to restore rejection, while in the first few weeks post-transplant, both CD4+ and CD8+ T cells from CSA-treated hosts have the capacity to effect PVG graft rejection. In this study, we demonstrated the CD4+ suppressor cells also had the capacity to inhibit restoration of rejection by CD4+ cells from CSA-treated DA rats recently transplanted with PVG hearts, and from rats sensitized to third party, but not from those specifically sensitized to PVG. They also inhibited the capacity of both naive CD8+ and sensitized CD8+ cells to effect rejection. These results showed that the CD4+ suppressor cell was capable of overriding the capacity to effect rejection of the CD4+ cell and activated CD8+ cells that were present in the CSA-treated host shortly after transplantation. The failure of naive CD8+ cells to augment suppression and the capacity of CD4+ suppressor cells to transfer unresponsiveness to irradiated hosts in which regeneration of CD8+ cells was abolished by thymectomy suggested that it was the CD4+ cell alone that mediated suppression. However, the failure of CD4+ suppressor cells to reinduce unresponsiveness in irradiated hosts whose CD8+ cells had been depleted by therapy with the mAb MRC Ox8 showed that a radioresistant CD8+ cell was required to reestablish the state of specific unresponsiveness. The induction of CD4+ suppressor cells in thymectomized hosts suggested that these cells were derived from long-lived CD4+ lymphocytes. However, their sensitivity to cyclophosphamide and their loss of suppressor function both after removal of the graft and after 3 d in culture demonstrated that the suppressor cell itself had a short lifespan. The CD4+ suppressor was shown to be MRC Ox22+ (CD45R+), MRC Ox17+ (MHC class II), and MRC Ox39+ (CD25, IL-2-R). These studies demonstrated the CD4+ suppressive cell identified in rats with specific unresponsiveness induced by CSA therapy had many features of the suppressor inducer cell identified in in vitro studies of the alloimmune response.(ABSTRACT TRUNCATED AT 400 WORDS)