Using a panel of B-cell antibodies recognizing clusters of leucocyte differentiation antigens, immunostaining patterns of eight reactive lymph nodes and 28 centroblastic/centrocytic and centrocytic lymphomas have been studied. Centroblastic/centrocytic and centrocytic lymphomas retained many of the B-cell differentiation antigens and neoplastic follicles partially recapitulated the staining patterns observed in reactive follicles. Centrocytic lymphomas usually expressed a heavy chain mantle zone-like phenotype. Nearly one-half of follicular lymphomas showed extension of neoplastic cells into interfollicular areas as evidenced by positivity for CD10 (common acute lymphoblastic leukaemia) and/or CD9 (immature B-cell) and CD23 (B-blast cell) antigens. Cases showing interfollicular involvement also manifested considerable phenotypic heterogeneity. Light chain restriction could not be used to determine interfollicular involvement because of the presence of many non-neoplastic cells. Most follicular lymphomas retained a polyclonal mantle around at least some neoplastic follicles and in no case was a monoclonal mantle seen. Most lymphomas (16/21) were diploid when examined by flow cytometry. Diploid tumours exhibiting interfollicular lymphomatous involvement had high proliferation (S + G2) fractions and these lymph nodes were usually derived from patients with widespread disease. Tumours containing a high percentage of cells in the G0/G1 phases displayed fewer B-cell differentiation antigens than tumours with low G0/G1 fractions.