Abstract Four human renal allografts were removed during the primary acute rejection because of transplant rupture. The transplants were perfused from blood and disaggregated into single cell suspension with collagenase-DNAse. The low red cell/white cell ratio in the dispersate, compared to blood, demonstrated that blood contamination of the dispersates was minimal. Cytological and subclass analysis of the infiltrating ceils was performed from cytocentrifuged preparations of the initial dispersate. Approximately 26% of the allograft-infiltrating cells were tissue macrophages, 24% lymphocytes, 19% monocytes, and 18% polymorphonuclear leukocytes. Lymphoblasts (7%) and plasmablasts plus plasma cells at various stages of maturation (5%) were also present in the infiltrates. Seventy-four percent of the infiltrating lymphocytes were SRBC-binding and acid α-naphthyl acetate esterase (ANAE) marker-expressing (T) cells, and 14% SIg-carrying (B) cells. Most (83%) of the infiltrating macrophages, 47% of the infiltrating monocytes and 23% of the infiltrating lymphocytes expressed the Fc-receptor for IgG. The infiltrating cells and transplant parenchymal cells were fractionated apart by 1 g velocity sedimentation. The infiltrating cells and blood mononuclear leukocytes of the transplant recipients were then tested for in vitro cytotoxicity to cultured donor spleen-derived and irrelevant macrophages, and to the parenchymal cells of the transplant. The infiltrating cells were far more cytotoxic to donor-derived macrophages than were blood leukocytes, and the cytotoxic effect was specific. In contrast, the blood leukocytes and the infiltrating cells killed extremely weakly, if at all, the (relevant) transplant parenchymal cells. This suggests that other mechanisms than direct contact-mediated cellular cytotoxicity are partially responsible for the deterioration of the graft function during allograft rejection.