The radioprotective capacity of folinic-acid (58059) (FA) was determined by measuring the reduction of radiation induced chromosomal damage as measured by micronuclei and structural chromosomal aberrations (SCA). V79 Chinese-hamster lung fibroblasts were irradiated with 1, 2, 4, 8, or 12 gray (Gy) of X-rays, or 50, 100, 200, 400, or 800 microJoules of ultraviolet (UV) rays. FA was given at 5 or 50 micrograms/milliliter after irradiation. X-rays were cytotoxic to V79 cells. The percentage of micronucleated binucleated (MNBN) cells increased in a dose dependent manner. The most common aberrations were dicentrics, chromatid breaks, and triradials. At the higher X-ray dose, 92% of cells showed some aberration. UV irradiation increased the MNBN in a dose dependent manner, with a 6.8 to 20.6 fold increase in MNBN at various doses. SCA also increased in UV treated cells, and minutes, chromatid breaks, and double minutes were common types of aberrations. FA significantly decreased the percent of MNBN cells at the 50 microgram level for all doses of X-rays. The percent inhibition ranged from 11.8 to 46.3%. As the X-ray dose increased, the percent inhibition decreased. At 5 microgram FA, reduction of X-ray damage occurred only at 4 and 12Gy, ranging from 0.94 to 16.6%. Aberrations decreased 50% both at 2 and 12Gy with 50 micrograms FA. In UV treated cells, 5 micrograms FA did not reduce the percent MNBN. At 50 micrograms FA, percent MNBN decreased significantly, ranging from 23.6 to 34.7% inhibition. FA did not affect SCA at either dose. The percent aberrant cells was reduced significantly for all UV doses when 50 micrograms FA was given. The authors conclude that FA is anticlastogenic against X-ray and UV induced chromosomal damage primarily at 50 micrograms/milliliter, but its exact mechanism requires further investigation.