Within 3-6 hr after the reestablishment of the circulation, a characteristic pathology developed in renal homotransplants. Blood monocytes and lymphocytes adhered to large thin-walled vessels of the septa carrying interlobular arteries, traversed their walls, and aggregated in the connective tissue spaces around them. Within 3 days, the number and size of the extravascular cells markedly increased, filling the septa completely and spreading from them centrifugally to occupy the intertubular spaces throughout the cortex. The composition of these aggregates at first was a mixture of lymphocytes and monocytes, and later consisted of large blast cells, macrophages, a few plasma cells, and polymorphonuclear leukocytes. Mitotic activity was seen 2 days after surgery among the large blast cells and increased to a maximal level a day later. Coevally with these changes, the thin-walled septal vessels, intertubular veins and capillaries, and finally, arteries and arterioles, in that order, were damaged. Focal injury of tubules was slight 24 hr after homografting; widespread cortical necrosis had developed 5-7 days later. At no time up to 7 days were concentrations of immunoglobulins detected by fluorescence microscopy in the transplanted kidneys. The morphologic manifestations and temporal sequences of renal homograft destruction suggested that several mechanisms acted synergistically to eliminate the transplant. The initial injury appeared to be the result of an interaction between host lymphoid cells and target endothelium, a phenomenon akin to allogeneic inhibition; followed by spreading ischemia; additional contact injury to tubules; and nonspecific inflammation associated with necrobiotic tissue.