Radiation-induced Long-Lived Radicals(LLRs), assigned as sulfinyl radicals (SLF: R-S-O•), in mammalian cells are likely to be mutagenic because scavenging LLRs by addition of Vit. C AFTER irradiation is correlated to the reduction of mutation frequency. In this study, we have performed UVB radiation to human fibroblast (HE40) cells, and hprt- mutation is analyzed by 6-thioguanine (TG) selection and multiplex PCR methods. UVB-induced LLRs in Syrian hamster embryo cells were also measured by ESR. The result of the 6-TG method indicates that levels of HPRT- mutation by UVB radiation are significantly higher than that of control, and were reduced to 40% by addition of Vit. C at 20 min AFTER irradiation. PCR analysis indicates that most of mutants are point mutation-type and total deletion or partial deletion-type are rarely found, so that most of mutation types are point mutations. LLRs measurements and analysis indicate that there are two kind of radicals as methyne disulfide radicals (MDS: R-S-CH•-S-R') and sulfonyl radicals (SFO: R-SO₂•), with ESR parameters of hyperfine coupling constant by one proton = 1.76 mT, g = 2.003 for MDS, and maximum slope width = 2.0 mT and g = 2.010 for SFO, respectively. MDS radicals yielded in 88 % of all LLRs, have long half-lifetime of 4 h, and were scavenged by Vit. C., so that MDS radicals are likely to be mutagenic. Chemical species of mutagenic LLRs are different as MDS and SLF radicals for UVB and ionizing radiations, respectively.