The role of suppressor cells was tested in vivo by passive transfer of lymphoid cells from enhanced rats bearing well functioning cardiac allografts into unmodified syngeneic animals receiving test heart grafts 1 day later. LEW rats were enhanced with 107 BN splenocytes and 1 ml LEW anti-BN antiserum 11 and 10 days, respectively, before receiving a (LEW × BN) F1 heart graft. All such grafts function over 25 days; one-third indefinitely. Grafts in unmodified hosts survive 7 ± 0.3 days. Thymocytes (108) harvested from enhanced recipients 6 days after grafting and transferred to unmodified LEW animals prolonged test graft survival to MST of 17 ± 3 days (p < 0.001). Splenocytes (nylon wool-adherent and nonadherent fractions) from such animals prolonged test graft survival to 12 ± 4 and 14 ± 4 days, respectively. Transfer of thymocytes from immunologically virgin LEW rats or LEW rats bearing acutely rejecting (LEW × BN) F1 grafts did not prolong test graft survival. Specificity was assessed by transferring cells from enhanced LEW recipients of (LEW × BN) F1 heart grafts into LEW recipients of WF grafts. WF graft survival was not prolonged. Since increased graft survival in unmodified hosts is a stringent requirement for the effect of suppressor cells in vivo, we conclude that such cells participate in the maintenance of immunologic enhancement.