The amounts of unscheduled DNA synthesis in fibroblasts originating from 34 species in 11 orders of mammals, after 10Jm^-2 UV irradiation, were assayed by autoradiography. Genome size of the species was determined by cytophotometric measurements of DNA contents of the cultured cells and the ability of unscheduled DNA synthesis per unit amount of DNA was computed. There was no correlation between repair potentials and life span, denying the presence of the correlation suggested by Hart and Setlow on 7 species.