Rat renal allograft survival is markedly prolonged by passive enhancement. Radio-labelled AS-anti-HO antigen-reactive cells (*ARC) were prepared by immunizing AS rats with HO rat tissue and injecting 125IUdR over a period of 4 days. Most of the radioactivity in spleen cells from these rats were associated with antigen-reactive cells which formed rosettes specifically with HO erythrocytes. *ARC were specifically opsonized in syngeneic AS rats bearing enhanced (AS times HO)F1 kidney allografts but not in untreated, rejecting allografted rats. *ARC incubated in vitro with serum from enhanced rats and injected into normal animals were opsonized whereas serum from rats rejecting grafts was ineffective. *ARC incubated in vitro with hyperimmune AS-anti-HO serum were not opsonized, unless the antiserum was first incubated with HO tissue under conditions favouring the formation of immune complexes. These results are consistent with the hypothesis that ARC bind antigenic determinants of antigen-antibody complexes and are opsonized by virtue of the antibody Fc portion of the complexes.