Long-term survival of Ag-B compatible rat heart allografts was obtained by short-term treatment of the recipients with antilymphocytic serum (ALS). Graft survival apparently was based on a specific change in the hosts rather than on persistent nonspecific effects of ALS. The hosts were not fully tolerant in that they were able to reject secondary skin allografts from the heart donor strain, although in a delayed fashion. The long-surviving heart allografts retained their immunogenicity as they were rejected when retransplanted to new hosts. The passive transfer of serum from long-term heart graft acceptors to new hosts receiving fresh allografts delayed rejection by several days. This effect was seen only with the serum from long-term acceptors suggesting that serum-blocking factors were involved in long-term survival of the heart allografts. However, the ability of adoptively transferred lymphoid cells to break tolerance to a heart allograft residing in a classically tolerant host was tested. In contrast to normal lymphoid cells, cells from the long-term acceptors were unable to break tolerance, suggesting that a specific cellular tolerance had been induced in this cell population. Moreover, a serum from the long-term acceptors failed to block the breakage of tolerance by normal lymphoid cells.