The authors investigated the ability of 70 monoclonal antibodies obtained from the Third International Workshop on Human Leukocyte Antigens (Oxford, 1986) to mark T lymphocytes in B5-fixed paraffin-embedded tissue. No staining occurred with 65 of the antibodies; however, 5 antibodies marked small lymphocytes in the T-cell areas of human tonsil. Two antibodies which strongly labeled lymphocytes, UCHL1 and T2/48, were used to examine 106 cases of non-Hodgkin's lymphoma, 29 cases of Hodgkin's disease, and a variety of normal and neoplastic tissues. UCHL1 and T2/48 each marked 86% (37/43) of B5-fixed T-cell lymphomas. Only 50% of formalin-fixed T-cell lymphomas were marked with these antibodies. UCHL1 marked 1.8% (1/56) of the B-cell lymphomas, compared with T2/48, which marked 19.6% (11/56) of the B-cell lymphomas. T2/48 had the interesting attribute of marking cells of the follicular mantle-zone and intermediate lymphocytic lymphoma, suggesting that the antibody recognizes a B-cell differentiation antigen. No Reed-Sternberg cells, epithelial neoplasms, sarcomas, neurogenic tumors, or normal nonlymphoid tissue were marked by either antibody. These antibodies successfully mark T cells in paraffin tissue sections and should aid in the investigation and characterization of abnormal lymphoid proliferations, "undifferentiated" malignant neoplasms, and immunologically mediated disorders.