A 10-day course of cyclosporine treatment inhibits the capacity of DA rats to reject PVG heart grafts and leads to the development of specific unresponsiveness-and indefinite graft survival, which is mediated by a W3/ 25+(CD4+) suppressor cell. In this study the sequential changes in the alloreactivity of the CD4+and CD8+subsets of CsA-treated DA rats were examined. During the induction phase, 8 and 20 days posttransplant, W3/ 25+cells retained normal alloreactivity in that they adoptively restored PVG heart graft rejection in irradiated DA rats. By day 50 they had lost their capacity to restore rejection of PVG grafts but still retained their capacity to effect third party W/F graft rejection. W3/ 25+cells from control grafted rats adoptively restored PVG graft rejection even at 75 days posttransplant, suggesting that the loss of alloreactivity of W3/25+cells in CsA-treated rats was due to the prevention of rejection by CsA, andimot a consequence of sensitization to alloantigen. MRCOX8+cells from CsA-treated rats showed some evidence of sensitization at days 8 and 20 but lost this by day 50. These studies showed that during the induction phase, normal alloreactivity of W3/25+cells is retained and sensitization of MRGOX8+cells occurs. Specific loss of reactivity and suppressor potential of W3/25+cells developed later, when specific un-responsiveness to second donor strain grafts developed in these hosts.