Rabbits carrying skin allografts and autografts were killed at daily intervals up to 8 days after grafting, 2 hours after they were given an i.v. dose of tritiated thymidine. Counts of labelled epithelial cells in autoradiographs of grafts revealed a progressive increase in the rate of DNA synthesis in allografted skin, as the rejection process progressed, when compared to autografted skin in the same rabbit. It is possible that at concentrations below those that cause cell destruction the infiltrating lymphoid cells and/or transplantation antibody may produce epithelial cell proliferation.