Thymocytes were irradiated with fast electrons up to 6 Mrad in the presence and absence of oxygen. The cells were treated before irradiation with a cold shock to prevent any repair rejection during irradiation. The DNA isolated subsequently was analysed for double-strand breaks (dsb), actual single-strand breaks (ssb) and alkali-induced strand breaks (alisb). We observed a linear increase of all types of lesions with dose and an o.e.r. for dsb of 3-6, for ssb of 4-9 and for alisb of 2-1. The data do not deviate significantly from those, measured on thymocytes irradiated without cold shock. In DNA of irradiated thymocytes, the frequency of 3' and 5' hydroxyl and 5' phosphate end-groups was analysed enzymatically. In both the ssb and alisb, about 11 per cent of the terminals carry 5'OH end-groups and 20-40 per cent 5' phosphate groups. On the 3' terminals, 60-80 per cent of the ssb are identified as 3'OH end-groups, whereas on the alisb only a small amount of 3'OH end-groups if found. The frequency of characterized end-groups shows the same oxygen effect as the corresponding strand breaks. Therefore, in the presence and absence of oxygen, the same mechanism may be responsible for formation of DNA strand breaks in vivo.