Functioning Mononuclear cells have been harvested from heterotopic rat cardiac allografts during maximal transplant cellular infiltration. T cells, identified by a T cell-specific absorbed rabbit anti-rat brain serum, constituted two-thirds of the total cells recovered. Approximately 20% of the infiltrating cells bear and synthesize surface immunoglobulin. Macrophages, identified by latex ingestion and morphologic and cytochemical techniques, comprise 9% of the graft infiltrate. Donor-specific cytotoxic T lymphocytes are concentrated within the graft. A separate population of Fc receptor-positive recovered cells mediate antibody-dependent LMC (Ab-LMC). Neither effector cell was adherent or phagocytic. These studies have conclusively established that cytotoxic T lymphocytes accumulate within rejecting allografts; however, the enriched presence of cytotoxic T cells within the grafts is not fully dependent upon antigen recognition per se, since Lew animals grafted with both BN and BUF hearts have Lew anti-BN and Lew anti-BUF killer cells in each graft.