SUMMARYThe development of cellular infiltration in skin allografts was studied in mice by isolating the cells enzymatically. Lymphocytes, macrophage-like cells, neutrophils, reticular cells, “basophilic cells,” and mast cells were found. The absolute numbers of various cell types infiltrating a standard skin graft were counted 4, 6, 8, and 10 days after grafting. As controls, cells infiltrating 8-day-old isografts and those present in normal donor skin samples of similar size were isolated. A striking increase in the number of infiltrating cells, predominantly lymphocytes, was found between the 4th and 6th days after grafting. Infiltrating cells from an 8-day-old allograft resembled those from a corresponding isograft, but they were much more numerous in the former.