A novel antigen detected by the CBF.78 monoclonal antibody (MoAb) is strongly expressed on cortical thymocytes and weakly expressed on resting peripheral T lymphocytes. Expression of the antigen is increased on phytohemagglutinin (PHA)- and anti-CD3-activated T lymphocytes and on Epstein-Barr virus-transformed B lymphocytes. The CBF.78 immunoprecipitated a protein of 116 kD from resting and PHA-activated peripheral blood mononuclear cells. CBF.78 MoAb did not inhibit T-cell proliferation induced by anti-CD3 antibody. This MoAb was effective for immunostaining on paraffin sections after microwave-oven heating of tissue sections. Among malignant lymphomas, the antigen recognized by CBF.78 MoAb was found to be mainly expressed by T-cell lymphomas (49+ of 74), particularly those of high-grade malignancy (31+ of 36), whereas only occasional B-cell lymphomas (4+ of 107) expressed the antigen. A distinctive pattern of reactivity was shown by 108 cases of anaplastic large cell lymphomas. Strong positivity for CBF.78 antibody was observed in 86+ of 108 cases, irrespective of B, T, or null phenotype. This multicenter study suggests that CBF.78 MoAb could be of diagnostic value in differentiating Hodgkin's-like anaplastic large cell lymphomas from cases of Hodgkin's disease rich in neoplastic cells. Only a few cases of Hodgkin's disease (13+ of 126) showed rare Reed-Sternberg cells that stained, In these few cases, staining was weak to moderate and confined to cytoplasm. CBF.78 MoAb was nonreactive with all nonhematopoietic neoplasms examined (0+ of 48). Further studies should delineate the function of this new antigen and its clinical utility.