Antisera were prepared in rabbits by immunization with either human peripheral blood T lymphocytes (PTL) or tonsil B lymphocytes (TBL). Specificities of sequentially absorbed antisera were examined by cytotoxicity testing, immunoperoxidase staining and absorption study. Anti-PTL serum (ATS) reacted with E rosetting lymphocytes in the normal population. ATS was also reactive with some of lymphoid neoplasms, whether neoplastic cells carried E receptors or not. This serum did not lyse any of myelogenous leukemia cells. When antiserum was used in the immunoperoxidase staining, ATS strongly stained paracortical cells in the lymph node and medullary thymocytes in the thymus. On the other hand, anti-TBL serum (ABS) was reactive with lymphocytes corresponding to the percentage of EAC binding lymphocytes in the normal population. ABS reacted not only with the neoplastic cells carrying C3 receptors and/or surface immunoglobulin, but also with non T . non B-acute lymphoblastic leukemia cells and a small number of acute myelogenous leukemia cells. In the immunoperoxidase staining, lymph follicular cells showed positive immunoreactivity with ABS, but paracortical cells did not.