Chinese hamster ovary cells (CHO) were X-irradiated in G1 and G2 stages of the cell cycle and subsequently Neurospora endonuclease (NE) (E.C.3.1.4), an enzyme which is specific in cleaving single-stranded DNA, was introduced into the cells, after making the cells permeable by treatment with inactivated Sendai virus. With this treatment all classes of X-ray-induced chromatid aberrations increased in G2 cells, whereas in G1 cells an increase in chromosome type of aberrations was found, associated with a profound induction of chromatid type of aberrations as well. Duration of the availability of single-strand gaps for the action of NE has been studied in G2 cells following X-irradiation and the influence of different parts of the G2 stage on the type and frequencies of chromatid aberrations was discerned. While the increase in chromosome type of aberrations by NE in X-irradiated G1 cells has been interpreted as due to the conversion of DNA single-strand breaks or gaps to double-strand breaks by NE, the induction of chromatid aberrations in G1 has been assumed to be due to conversion of some of the damaged bases into strand breaks by NE. Biochemical evidence is presented for the conversion by NE of DNA single-strand breaks induced by X-rays into double-strand breaks using neutral sucrose gradient centrifugation.