Abstract Folinic acid (FA), clinically called leucovorin, has been widely used as a nutrient supplement in dietary intake and is capable of inhibiting cytotoxicity and chromosomal damage induced by chemicals. However, data on its antigenotoxic effect on radiation-induced chromosomal damage are limited. The present study was, therefore, performed to investigate the effect of FA on radiation-induced (X-rays and UV radiation) micronuclei (MN) and structural chromosomal aberrations (SCA) concurrently in V79 Chinese hamster lung cells. Exponentially growing cells were exposed to five doses of X-rays (1–12 Gy) and UV radiation (50–800 μ × 10 2/cm 2) and post-treated with 5 or 50 μg FA/ml of culture medium for 16 h. The slides were analyzed for the presence of MN and SCA using standard procedures. The results showed that X-ray treatment alone produced dose-related cytotoxicity as measured by nuclear division index (NDI) and mitotic index (MI). X-rays produced a clear dose-related clastogenicity as measured by percent of micronucleated binucleated cells (MNBN) (5–79%) and percent of aberrant cells (11–92%). FA at 5 μg/ml slightly decreased X-ray induced chromosomal damage in both assays; however, the inhibition was significant (12–46% of MNBN, 14–48% in aberrant cells) only when X-ray-treated cultures were post-treated with 50 μg FA/ml. Post-treatment of FA had no effect on X-ray induced cytotoxicity as measured by NDI and MI. A similar a dose-related increase in % MNBN (0.5–10.3%) and percent aberrant cells (6–35%) was produced by UV radiation treatment alone. There were significant percentages of MNBN and aberrant cell inhibitions at both 5 and 50 μg/ml in both assays. As in the case of X-ray-treated cells, there was a clear dose-related cytotoxicity in UV-treated cells alone. No reduction in NDI or MI was found when UV-exposed cells were post-treated with 5 or 50 μg of FA. These data demonstrate the beneficial effect of FA in decreasing radiation-induced chromosomal damage.