AbstractThe immunological properties of two murine thymus‐dependent (T) lymphocyte surface glycoproteins, T200 and T25, were investigated. T200 is a lymphocyte‐specific antigen with a high degree of species specificity. It shares antigenic determinants with molecules present on thymus‐independent (B) lymphocytes.T25 has antigenic determinants which cross‐react with antigens on mouse brain, rat thymocytes and rat brain. An antiserum against a purified rat brain glycoprotein which carries Thy‐1.1 reacts with T25. Absorption of this antiserum with BALB/c thymocytes or brain homogenate produces a Thy‐1.1 ‐specific serum which reacts with T25 from AKR/J thymocytes but not with T25 from AKR/Cum thymocytes. These results confirm that T25 is the molecule on the surface of mouse T cells which carries the Thy‐1 antigen. T25 also carries antigenic determinants, recognized by anti‐thymocyte serum (ATS), which were found on secondary mouse embryo fibroblasts and untransformed fibroblast cell lines but which were not detected on fibroblast cell lines transformed with murine sarcoma virus (MSV) or with Simian virus 40 (SV40).A third T lymphocyte‐specific antigen with an apparent mol. wt. of 35 000 Daltons, that is recognized by ATS and is expressed on thymocytes and spleen cells, was detected. In contrast to T25 this molecule is not present on brain nor on BW5147, a T cell lymphoma.