The kinetics of chromatid aberrations have been studied in human lymphocytes exposed to X-rays in the G₂ phase of the cell cycle and incubated with or without the nucleoside analogue 9-β-D-arabinofuranosyladenine (ara A), known to inhibit the repair of DNA double-strand breaks. In the absence of ara A an exponential decrease in frequencies of chromatid breaks occurred which we interpret as repair. Few breaks were observed if samples were harvested immediately following irradiation. The frequency of chromatid breaks at 1 h after X-irradiation (442 per 100 cells/Gy) was similar to that previously observed in Chinese hamster ovary (CHO) K1 cells. However, the exponential decrease of chromatid breaks between X-irradiation and sampling occurred with a t_1/2 of 0.87 h, a faster rate than we have previously observed in CHO K1 cells and was not inhibited by 200 μM ara A alone, in contrast with our previous findings in a human fibroblast line. However, in the presence of the ADA inhibitor coformycin, inhibition of break repair was already observed at an ara A concentration of 100 μM indicating that the apparent unresponsiveness to ara A of lymphocyte chromatid break rejoining results from the deamination of this nucleoside analogue. This deamination effect was confirmed by measurements of DNA synthesis which showed stable inhibition of synthesis by ara A only when coformycin was present. Frequencies of chromatid exchanges in irradiated cells remained constant except at the sampling time directly after irradiation, consistent with the view that chromosomal radiosensitivity remained constant throughout the G₂ phase, except for the period immediately prior to mitosis. The overall mean frequencies of exchanges was 2.48 per 100 cells/Gy, lower than that in human fibroblasts or CHO K1 cells, rising to 6.27 per 100 cells/Gy after incubation in the presence of ara A and coformycin between irradiation and sampling.