A leukocyte-common (L-C) antigen which can be dominant as an immunogen in rabbit anti-rat thoracic duct lymphocyte serum has been purified from rat thymocytes. Initially, an antigenic fragment of 100,000 apparent mol. wt. was prepared at 400 to 900-fold purification by lentil lectin affinity chromatography and gel filtration in deoxycholate. Mice were then immunized with this fraction, and a hybrid myeloma cell line secreting antibody to the L-C antigen was prepared by cell fusion. This antibody was used for affinity chromatography and gave pure L-C antigen at 1400-fold purification compared with thymocytes. The L-C antigen is a major membrane glyco-protein of rat thymocytes and has an apparent mol. wt. of 150,000 as determined by electrophoresis on polyacrylamide gels in sodium dodecyl sulfate. The antigen constitutes one of the three thymocyte glycoproteins which stain intensely for carbohydrate with periodic acid Schiff stain. It is present on greater than 95% of thymocytes, bone marrow cells and thoracic duct lymphocytes.