Both the xrs and V-3 lines of Chinese hamster ovary cells exhibit marked sensitivity to ionizing radiation. They are also sensitive to agents such as bleomycin and H₂O₂ but exhibit normal responses to ultraviolet light and mitomycin C. Both cell lines are defective in split-dose repair and repair of double-strand breaks in DNA. Analysis of response to radiation as a function of age in the cell cycle indicates that both cell lines exhibit a marked sensitivity in late G₁ and early S phase with more limited sensitization throughout the remainder of the cell cycle.