A human lung tumor-associated antigen (LTA), previously isolated from a small cell carcinoma, was further studied after labeling with N-succinimidyl-3-(4-hydroxy-5-[125I]iodophenyl)propionate. Two immunoreactive forms of the labeled antigen were observed and isolated after electrophoresis in 7% polyacrylamide gels. These components, referred to as LTA-I and LTA-II in order of mobility, were judged homogeneous by gel electrophoresis, G-200 gel filtration, size exclusion high-performance liquid chromatography and sedimentation velocity analysis. The latter 3 techniques produced identical profiles for both forms of the LTA. Sephadex chromatography and high-performance liquid chromatography analyses indicated the mass of the antigens to be .apprx. 140,000-150,000 daltons with a D20,w of 4.2-4.3 .times. 10-7 cm2/s. The S20,w values for both were 4.5-4.6S. Sodium dodecyl sulfate gel electrophoresis of LTA-II gave a single component with a MW of 81,700, while LTA-I had a major component identical in size to LTA-II and 2 minor components with MW of 50,000 and 27,700, respectively. The isoelectric point of LTA-II (peaks at pH 2.6 and 3.2) generally was more acidic than LTA-I (major component centered at pH 4.7, minor component centered at pH 3.1). A radioimmunoassay, with a useful detection range of from 1 to 100 ng/ml, was developed with LTA-I. This assay was used to determine the concentration of LTA in the sera of normal and lung cancer patients. Fifteen normal sera had a mean of 17 .+-. 22 (SD) ng/ml, with none greater than 83 ng/ml (+3 SD). Thirteen lung cancer patients with stage I (i.e., localized disease) had a mean of 187 .+-. 219 ng/ml, with the means for 7 of 13 patients > 83 ng/ml; 15 lung cancer patients with stage III, more extensive disease, had a mean of 277 .+-. 252 ng/ml, with the means for 12 of 15 patients > 83 ng/ml. This antigen may be useful for the early detection or monitoring of lung cancer.