The murine thymus leukemia antigen (TL) has been solubilized from the tumor ASL1 and from an established cell line ASL1W, by papain digestion. When a 15-min digest was chromatographed on Sephadex G-200, two peaks of TL activity were eluted with apparent molecular weights of approximately 58,000 and 31,000. Chromatography of a 30-min digest under the same conditions resulted in elution of a single peak of activity with an apparent molecular weight of 58,000. Additional purification was carried out on the 58,000 molecular weight material by absorption to, and elution from DEAE-cellulose. The combination of gel filtration and ion exchange chromatography resulted in approximately a 150-fold purification. © 1975, American Chemical Society. All rights reserved.