An antiserum specific for human T lymphocytes (AMT) was used to examine patterns of T cell surface antigen expression and to isolate their reactive membrane antigens. By a quantitative adsorption assay, different plateaus of AMT reactivity with blood T cells were observed after serial adsorptions with individual T cell lines. MOLT-3 cells removed 95% of AMT activity to blood T cells whereas MOLT-4 removed 70% and HSB-2 removed only 30%. A cross-adsorption analysis demonstrated that each of the three cell lines differed in their adsorbing efficiency to remove AMT antibodies reactive with the reciprocal cell lines. Radiolabeled membrane proteins were solubilized with either sodium deoxycholate (DOC) or NP-40 detergents, precipitated with AMT, and Staphylococcus aureus Cowan strain I, and analyzed by sodium dodecyl sulfate-polyacrylamide disc gel electrophoresis (SDS-PAGE). Two distinct T lymphocyte antigens of approximately 25,000 daltons (p25) and 16,000 (p16) were identified on MOLT-3 cells. Similar relative quantities of p25 and p16 were detected on human peripheral T cells and thymocytes. MOLT-4 cells contained less of the p25 peak than did MOLT-3. HSB-2 cells gave a small peak in the same general location as the MOLT-3 p25 peak, and a relatively large p16 peak. Adsorption of AMT with HSB-2 removed the capacity of the antiserum to precipitate p16 from MOLT-3, but did not eliminate reactivity with p25.