Ninety-five diffuse large cell lymphomas in 89 patients were stained in cryostat sections with a panel of monoclonal antibodies. Lymphoma cells from 47 patients (53%) expressed either kappa or lambda light chains, usually in combination with mu heavy chains. Fifteen samples from 12 patients (14%) expressed two or more T-cell antigens and commonly expressed Ia antigens. Lymphoma cells from 10 of these patients uniformly lacked one or more pan T-cell antigens; lymphoma cells from 4 of these patients also lacked both T-subset antigens--findings which should prove useful in diagnosis. Lymphomas from 28 patients (31%) did not express immunoglobulin or T-cell antigens but commonly expressed the B-lineage antigen B1; and the remaining 9 cases generally expressed Ia antigens, common ALL antigens, or both. Our findings confirm the marked immunologic heterogeneity of diffuse large cell lymphomas; the phenotypic heterogeneity observed in T-cell cases in many instances is difficult to reconcile with current models of T-cell differentiation.