A human lung tumor-associated protein has been purified from an extract of a human small cell carcinoma of the lung and shown by Ouchterlony double diffusion analysis to be antigenically identical to a component which was previously demonstrated in 84 of 98 lung tumor extracts of all histological types but absent from extracts of normal adult and fetal lung, other normal tissues, and tumors of other organs. These studies utilized xenoantisera raised against a pool of lung tumor extracts which were exhaustively adsorbed with normal serum and tissue extracts. A radial immunodiffusion assay developed for the antigen permitted its quantitation throughout the course of isolation. Purification was accomplished by ion-exchange chromatography, gel filtration, and affinity immunoadsorption. By ion-exchange chromatography, the proteins appeared to be quite heterogeneous, with immunological reactivity detected in three different peaks. However, all the active components were immunologically identical. Gel filtration of the major antigenic component from diethylaminoethyl cellulose similarly demonstrated a further fractionation into several active, immunologically identical forms. These results suggest a charge-size isomeric relationship among the various forms, all of which possess a common and identical antigenic site. The major component was isolated throughout the purification scheme. The final product represented 9% of the input activity, produced a single, although broad, protein-staining region on 7% polyacrylamide gels which was coincident with antigenic activity, and exhibited immunological identity with the antigen in the crude extract as well as with that in an extract from another lung tumor.