The comparative clastogenicity and cytotoxicity of ultraviolet (UV) and X-radiation to V79 cells were examined. Exponentially growing Chinese-hamster-V79 lung fibroblasts were irradiated with X-rays at 100, 200, 400, or 800 rads, or 254 nanometer UV radiation at 100, 200, 400, or 800 microjoules per square centimeter (microJ/cm2). The cells were treated with cytochalasin-B or colcemid added 16 hours after exposure. The numbers of binucleated cells was determined. Cytotoxicity was assessed by determining the mitotic index (MI) and the nuclear division index (NDI) after treatment with colcemid and cytochalasin-B, respectively. The MI was determined from the number of metaphase cells and the NDI from the number of binucleated cells. Genotoxicity was assessed by scoring the cells for structural chromosome aberrations after colcemid treatment and for micronucleated binucleated (MNBN) cells following cytochalasin- B. X-Irradiation caused significant dose related decreases in MI and NDI and increases in the incidence of chromosome aberrations and MNBN cells. The NDI was decreased from 86.5% in control cells to 68% in cells irradiated with the 800rad dose. The 800rad dose decreased the MI by 50% compared to the control value. The proportion of MNBN cells and cells with chromosomal anomalies after exposure to 800rad X-rays was 65.7 and 69%, respectively, versus only 0.5% for both endpoints in control cells. Similar dose related changes in MI, NDI, and the frequency of MNBN cells and cells with chromosome aberrations were seen in UV irradiated cells, but the magnitude of the changes was smaller than in the case of X- irradiation. Induction of the cytotoxic and genotoxic effects by both types of radiation was well correlated with each other. The authors conclude that both X-rays and UV radiation induce clastogenicity and cytotoxicity. The amount of chromosome damage as measured by increases in the frequency of MNBN and aberrant cells appears to be well correlated with cytotoxicity as measured by decreases in NDI and MI.