Accessible surface proteins of thymus-derived lymphocytes (T-cells) of normal CBA mice and bone-marrow-derived lymphocytes (B-cells) of congenitally athymic nu/nu mice were analysed. The surfaces of lymphocytes were radioiodinated by using the enzyme lactoperoxidase (EC 1.11.1.7), then solubilized either in acid-urea or in the non-ionic detergent Nonidet P-40. These lysates were then precipitated with antisera specific to either immunoglobulin or the theta-alloantigen in order to assess the presence of these surface markers. Comparable amounts of radioactivity in proteins specifically precipitable as immunoglobulin were obtained from T-lymphocytes and B-lymphocytes when the cells were disrupted by acid-urea. This immunoglobulin had mol. wt. approx. 180000 and was composed of light chains and mu-type heavy chains. When radioiodinated lymphocytes were solubilized with Nonidet P-40, 3-4% of radioiodinated high-molecular-weight protein of B-cells consisted of immunoglobulin, a result similar to that found with acid-urea extraction. However, with the detergent extraction, only 0.1% of T-cell surface protein was precipitable by anti-globulin reagents. The theta-alloantigen was isolated from CBA T-cells both by acid-urea and by detergent lysis. This protein possessed a mobility on polyacrylamide-gel electrophoresis in sodium dodecyl sulphate which was consistent with a mol. wt. of 60000. An identical component was isolated from the theta-positive thymoma WEHI 105. The theta-antigen was not isolated from B-cells by either of the extraction procedures used. These results provide further evidence that the surface membranes of normal T-cells and B-cells differ in physicochemical properties. In particular, various surface components possess differential solubilities in non-ionic or organic solvents. This observation provides an explanation for discrepant results that have appeared in the literature concerning the isolation of immunoglobulin from T-lymphocytes.