Hybridomas were produced against the T-cell CLL derived-cell line, SKW3, by the fusion of hyperimmune spleen cells with P3 myeloma cells. One clone, designated DU-SKW3-1, was shown to produce a murine IgG2b antibody reactive with an antigen expressed on normal thymocytes and peripheral blood T cells. This antigen was not detected on human B cells, erythrocytes, monocytes, granulocytes, or platelets. D-SKW3-1 also reacted with T-ALL, T-CLL, and B-CLL cells, but did not react with common ALL or acute myelocytic or monocytic leukemias. Immunoprecipitation of lactoperoxidase-iodinated, detergent-solubilized PBL demonstrated that DU-SKW3-1 reacted with a protein with an apparent mass of 67,000 daltons (p67), which had identical mobility to the antigen precipitated by L17F12, Cocapping experiments suggested that DU-SKW3-1 and L17F12 detected the same molecule: however, DU-SKW3-1 was unable to block the binding of L17F12. In addition, DU-SKW3-1 reacted with the T lymphocytes of both the great apes and old world monkeys, in contrast to L17F12 and two other p67 monoclonals, T101 and 10.2, which reacted only with the cells of the great apes. This data suggests that DU-SKW3-1 may react with a second, less phylogenetically restricted epitope on the p67 T cell-/CLL-associated molecule.