The clinical, pathologic, and immunologic aspects of malignant lymphoma of centrocytic type (ML,cc) were studied at diagnosis and often at relapse in 18 patients. The typical patient was a middle-aged or older man with adenopathy, often massive splenomegaly, hepatomegaly, marrow involvement, and, not infrequently, peripheral blood involvement. Histopathologically, ML,cc had a diffuse or vaguely nodular growth pattern with, predominantly, cells resembling centrocytes (cleaved follicular center cells) sometimes with admixed small round lymphocytes but with virtually no transformed cells. In 2 cases the neoplastic cells formed a mantle zone around reactive-appearing follicles. Cell suspensions and frozen sections revealed the monoclonal B-cell nature of all but 1 nonmarking case, and the polyclonality of the follicles in the 1 mantle zone case tested. The B cells had some, but not all, characteristics of both normal mantle and follicular center cells when eight nodes were studied with the use of a panel of monoclonal antibodies, peanut lectin, and endogenous alkaline phosphatase activity. Of 13 patients who underwent repeat biopsies, 1 developed a high grade unclassifiable B-cell lymphoma, and 6 had less marked changes. None of 7 patients tested had a change in light chain class. In conclusion, ML,cc is a distinct entity separable from other B-cell lymphomas in which either centrocytes or small round lymphocytes predominate.