The age response for lethality of Chinese hamster cells to ultraviolet light shows that they are resistant in G(1), sensitive as they move into and through the S phase and resistant again in G(2) and mitosis. Survival curves determined at different times in the cycle reveal that mitotic cells are the most resistant fraction, much more resistant than S cells, and more resistant than either G(1) or G(2) cells. The extent to which the age response is ilfluenced by nucleic acid and protein synthesis was investigated by using inhibitors of these processes. In the presence of inhibitors of DNA or protein synthesis added to G(1) cells before exposure, cell survival neither declines to the minimum survival of S cells nor rises subsequently to the resistance of G(2) cells. If, before exposure, DNA synthesis is arrested in the middle of S, when survival is at a minimum, the subsequent rise in survival during G(2) is not prevented. However when cycloheximide is added before exposure, during the middle of S, this rise is prevented. When actinomycin D, an inhibitor of RNA synthesis is added prior to exposure the age response is affected only slightly. Postirradiation treatment of G(1) and mid-S cells with inhibitors of DNA or protein synthesis maintains survival at a level characteristic of the age of the cells.