Ionizing radiations induce mutations which can be detected both in coding sequences (Hprt locus) by measuring the frequency of 6-thioguanine-resistant cells and in minisatellite sequences by DNA fingerprint analysis. We analyzed the effects of irradiation with low-energy protons (31 keV/pm) and, for comparison, with ultraviolet light (254 nm), for which DNA damage and repair mechanisms are better understood, on cultures of Chinese hamster V79 cells with the two methods mentioned above. The results indicate that the frequency of 6-thioguanine-resistant cells was increased significantly, although very differently, by both treatments. The analyses carried out by DNA fingerprinting with a multilocus DNA probe show that the level of induction in minisatellite sequences was higher compared to those measured at the Hprt locus after proton irradiation, but lower after treatment with ultraviolet light.