The au component (120-150,000 d) of the human LT system has been purified to electrophoretic homogeneity. This form was obtained from Con A-stimulated human tonsil and adenoid lymphocytes. Purification was greatly facilitated by employing a low concentration of lactalbumin hydrolysate as a serum substitute. The scheme consisted of sequential molecular sieving, lectin-affinity chromatography on Con A-Sepharose, and isoelectricfocusing. The specific activity of the αHincreased approximately 5-10,000-fold in the course of purification, as judged by protein monitoring by the fluorescamine assay and radioiodination using the Iodogen technique. Homogeneity of the αHpreparation from electrofocusing was demonstrated by native PAGE, as only a single protein peak was observed. The identity of this radioative peak with the lytic moiety was apparent as it migrated to the only region of the gel where lytic activity was detectable. This represents the third human lymphokine purified to homogeneity. © 1981.