Specific anti-human thymus xenoantiserum (ATS) was utilized for characterizing a human thymus antigen (HTA) preferentially expressed on human thymocytes. Binding of ATS with different cell types was studied by immunofluorescence and immunoperoxidase techniques, as well as by radioimmunoprecipitation (RIP) followed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). ATS reacted selectively with the majority (87%) of human thymocytes among normal lymphoid cell populations tested. In the thymus, HTA was expressed largely on cortical thymocytes but not on those cells located in the thymic medulla. When 20 cases of different types of lymphatic leukemias were studied with ATS, HTA was found on acute lymphoblastic leukemia cells rosetting with sheep erythrocytes (T-ALL) but not on other lymphatic leukemia cases, including chronic lymphocytic leukemia cells rosetting with sheep red blood cells (T-CLL). SDS-PAGE analysis of immunoprecipitates formed between ATS and radiolabeled cell-surface components of human thymocytes demonstrated that ATS bound a 48,000-molecular-weight component that could be adsorbed to Sepharose 4B coupled with Les culinaris hemagglutinin and could be labeled by periodate-tritiated borohydride, indicating that HTA is a sialoglycoprotein. Similar antigen molecules were also precipitated by ATS from T-ALL cells but not from other lymphatic leukemia cells. Five T-ALL-derived cell lines were tested with ATS by immunofluorescence and by RIP and SDS-PAGE, but only two cell lines (MOLT-3 and P12/Ich) possessed HTA molecules on their cell membrane.