Two monkey antisera against human thymocytes after absorption with human erythrocytes and peripheral blood leukocytes were shown to detect human thymus-leukemia (HTL)-like antigens. These sera were cytotoxic for thymocytes (> 90% lysis at a 1:10 dilution) but were nonreactive with enriched peripheral blood T- and B-lymphocytes or with cells from myeloid or B-cell lymphoid leukemias. Most (16/17) sheep erythrocyte rosette-forming acute lymphoblastic leukemia (ALL) cells reacted with these sera. Cells from patients with T-cell chronic lymphocytic leukemia, lymphoblastic lymphoma (LBL), and thymoma were also positive. Three of 4 T-cell lymphoblastoid lines derived from ALL patients reacted with these sera. Absorption of the sera with MOLT-4F cells, thymocytes, or LBL cells removed the reactivity against all types of cells tested. However, sera absorbed with the T-cell line HSB remained cytotoxic for thymocytes, MOLT-4F, and most (6/9) T-cell cancers tested. The peripheral blood cell-absorbed sera precipitated a molecule with an apparent molecular weight of 48,000 from lactoperoxidase-labeled thymocytes but not from similarly labeled peripheral blood lymphocytes. The ability of the sera to precipitate this antigen was decreased by absorption with thymocytes, MOLT-4, or LBL cells but not by absorption with HSB, SB, or non-T, non-B ALL cells. Sequential precipitation studies suggested that the HTL antigen was not associated with beta 2 microglobulin.