The human shuttle plasmid pZ189, containing the Escherichia coli supF gene as the mutational target, was irradiated in vitro with 210Po alpha particles and transfected into human lymphoblastoid cells. Plasmids which were replicated in human cells were recovered and those containing mutant supF genes were isolated by phenotypic screening in E. coli. The mutations were characterized by sequencing the tRNA gene. The mutant frequency increased linearly with the alpha-particle dose and, at 259 Gy, it was 16 times (0.29%) that observed in unirradiated controls (0.018%). The distribution of alpha-particle-induced point mutations was highly nonrandom and similar to that observed in the unirradiated or X-irradiated plasmid DNAs. The majority of the mutations were G.C----A.T transitions and occurred selectively at most 5'-TC (3'-AG) and 5'-CC (3'-GG) sequences. For the unirradiated control DNA, these mutations at C's (G's) were preferentially located in the nontranscribed strand, similar to the observation previously made for mutations in X-irradiated DNA. Such a strand bias was not observed for mutations in the alpha-particle-irradiated DNA. The data suggest that, although similar types of point mutations are induced in unirradiated, X-irradiated, and alpha-particle-irradiated DNAs, the mechanisms of their induction and the exact nature of the lesions involved may be quite different.