We have measured clonal survival following gamma-irradiation of unstimulated (G0) T-lymphocytes from 35 donors, of 11 T-lymphocyte cell lines, of six lymphoblastoid cell lines, and of nine primary fibroblast strains for which we have G0 T-lymphocyte material from the same donor. Amongst the G0 lymphocytes we have results from nine normal donors, from eight cord bloods, from seven ataxia-telangiectasia (A-T) patients and from nine A-T heterozygotes. Although there is some variation between samples, G0 T-lymphocytes from normal donors appear to be slightly more radioresistant than T-lymphocyte lines, with a more shouldered survival curve. From our limited sample, lymphoblastoid cell lines appear to be slightly more radiosensitive than T-lymphocytes. The overall radiosensitivity of primary fibroblasts appears to be broadly similar to that of G0 T-lymphocytes. In nine instances, five A-Ts and four A-T heterozygotes, both G0 T-lymphocytes and primary fibroblasts from the same donor were tested. In five cases there was closely similar radiosensitivity in the two cell types, but in four cases there was some discrepancy. Further work, especially with normal donors, will be required in order to establish how reliably radiosensitivity in other cell types can be predicted from that of G0 T-lymphocytes. In all cell types the hypersensitivity of A-T cells was confirmed. Furthermore, the marginally greater sensitivity of A-T heterozygotes, when compared as a group with normals, was confirmed with G0 T-lymphocytes. Our results also suggest a slightly increased radiosensitivity in G0 T-lymphocytes from some, but not all, cord blood samples.