Mutant frequencies in peripheral blood lymphocytes and erythrocytes have been measured for atomic bomb survivors having various DS86 doses. Among the four assay systems utilized so far, erythrocyte glycophorin A assay revealed a dose related increase of mutant frequency, similar to the results obtained with in vitro mutagenesis studies at HPRT locus using human diploid cells. Mutant T-lymphocyte frequency of the HPRT locus detected as resistant to 6-thioguanine significantly increased with DS86 dose however the slope was very shallow compared with that of in vitro study. Mutation at T-cell receptor genes and the HLA-A gene did not show a significant increase in frequency with dose in cells of survivors studied.