Chemical carcinogenesis in the regenerating rat liver is cell-cycle-dependent. Proliferating hepatocytes were maximally susceptible to initiation by a single dose of benzo[a]pyrene diolepoxide I when at the G1/S border. Hepatocytes in early G1 or late S/G2/M were less susceptible and non-proliferating G0 hepatocytes were resistant to initiation. Radiation clastogenesis in proliferating human fibroblasts also is cell-cycle-dependent. Ultraviolet radiation (UV) induced maximal frequencies of chromosomal aberrations in synchronized cells that were at the G1/S border. Cells in early G1 or G2 were significantly less sensitive. For both initiation of chemical carcinogenesis and UV-clastogenesis, it appears that replication of damaged DNA is required and DNA repair before replication reduces cellular risk. If DNA repair is protective, cell cycle checkpoints which delay DNA replication and mitosis should augment this protective influence by providing more time for repair. The contribution of cell cycle checkpoint function to DNA repair during cell cycle-dependent clastogenesis was studied using ataxia telangiectasia (AT) fibroblasts. The AT cells displayed a defect in the coupling of DNA damage to checkpoints which control the G1/S and G2/M transitions and the rate of replicon initiation in S phase cells. UV-clastogenesis in AT cells was cell-cycle-dependent with irradiation at the G1/S boundary inducing 3-times more aberrations than treatment in G0 at the time of release into the cell cycle. Thus, DNA excision repair during the pre-replicative G1 phase was protective even in cells with defective checkpoint function. However, following irradiation at the G1/S border, AT cells displayed about 6-fold increased levels of UV-induced chromosome aberrations in comparison to normal human fibroblasts that were treated at this time. These observations indicate that secondary and tertiary DNA lesions that are produced during replication of UV-damaged DNA (replicative gaps and double-strand breaks) also depend on checkpoint function for repair. The replicon initiation and G2-delay checkpoints that operate after initiation of S phase appear to play a major role in protection against UV-clastogenesis.