Mouse cells (3T3 line) excised at most 20% of the pyrimidine dimers introduced into their DNA by a dose of short-wavelength ultraviolet radiation that allows a significant fraction of the cells to survive. When irradiation was delivered at the pre-replicative stage, a significant repair of lethal events was observed, as the cells progressed toward S phase. The recovery in survival cannot be accounted for solely by excision of pyrimidine dimers. Therefore, either another lesion produced by ultraviolet radiation is critical in terms of lethality, or the dimer, which may trigger the lethal event, becomes no longer an obstacle for the replication system after a certain period of time.