In the course of analyzing human lymphoma tissue with conventional surface marker techniques and with monoclonal antibodies directed against T cell subsets, five tumors were encountered with dual B and T cell determinants. All bore on their surface membrane IgM of kappa light chain type, complement receptors, and the Ia-like antigen. In each of the five cases, the neoplastic lymphocytes reacted with a monoclonal antibody that detects the sheep erythrocyte receptor (OKT11); all but one reacted with a monoclonal antibody for peripheral T cells (OKT3); and all but one reacted with a monoclonal antibody specific for either the inducer-helper (OKT4) or the cytotoxic-suppressor (OKT8) T cell subsets. In addition, lymphocytes from two of the five cases formed spontaneous rosettes with sheep erythrocytes (E-rosettes). These tumors with dual B and T surface characteristics were confined to human malignant lymphomas that originate from B lymphocytes of the follicle center.