Lymphocytes from a common human leukemia, chronic lymphocytic leukemia, have a greatly enhanced capability of DNA repair and a concomitantly prolonged survival in vitro after damage to DNA. From these lymphocytes, the authors isolated and purified a DNA binding protein with a molecular weight, of 24,000. It binds tightly to ultraviolet light (UV irradiated and single stranded DNA. At 35° it enhances the helix coil transition of poly[d(A T)] and the UV irradiated calf thymus DNA but is inefficient in ordinary native DNA. This protein also facilitates the rate of UV endonuclease incision of UV DNA but does not induce any nicks by itself. This finding suggests that the protein may be involved in DNA repair by enhancing such activity, and also offers an explanation for the observation of increased DNA repair in chronic lymphocytic leukemia cells. When human metaphase chromosomes are exposed to the protein, it induces marked lengthening of chromatids suggesting that this protein may also act on complex chromosomes. By quantitative immunochemical determinations, such protein could not be found in lymphocyte extracts of three normal individuals.