TK6, WI-L2, SB and three other B-lymphoblast lines were deficient in the rejoining of DNA double-strand breaks (DSBs) induced by ionizing radiation. Cells of these cell lines rejoin less than 50% of the breaks in 60 min after exposure, as assayed by filter elution at pH 9.6. The deficiency in TK6 cells was confirmed using the comet assay. IN TK6 cells the percentage of DSB rejoining did not vary markedly with dose and was similar for G1, S, and G2 + M-phase cells. Two B-lymphocyte lines (Raji and GM0606), three T-lymphoblast lines (MOLT-4, Jurkat, and CCRF-HSB-2), HL-60 promyelocytes, and GM3440 human skin fibroblasts rejoined more than 50% of the DSBs in this period after exposure. Radiation sensitivity in terms of cell survival was measured in those cells forming colonies. Of the cell lines tested, those that were deficient in DSB rejoining were radiation-sensitive (TK6 and WI-L2: D0 = 0.64 Gy). However, not all lines that were proficient in DSB rejoining were radiation-resistant, since Jurkat and GM0606 cells were relatively radiation-sensitive (D0 = 0.63-0.73 Gy). TK6 and WI-L2 cells were more sensitive to bleomycin (D0 = 8-9 micrograms/ml) than were HL-60 and Raji cells (D0 = 40-54 micrograms/ml). No relationship of DSB rejoining to V(D)J recombinase activity was observed, since no mRNA hybridizing to the cDNA probes for RAG-1 or RAG-2 was detected in any of the cell lines tested.