Two monoclonal antibodies (FB1 and FB21) reactive in formalin-fixed, paraffin-embedded tissue sections are reported in this paper. FB1 and FB21 recognize a cytoplasmic antigen and a surface antigen of B cells, respectively. FB1 reacts with mantle zone (MZ) B cells, germinal centre (GC) cells, and marginal zone (MrZ) B cells, but not with T cells in lymphoid tissues. FB21 reacts with MZ B cells, GC cells in lymphoid tissues, and T cells of peripheral blood, but not with MrZ B cells in the spleen. Neither monoclonal antibody (MoAb) reacts with monocytes, granulocytes, or plasma cells. FB1 reacted with all the B-cell lymphomas tested and with CD20-positive Reed-Sternberg cells in two of five cases of Hodgkin's disease, but not with multiple myelomas or T-cell lymphomas. FB21 reacted with B-cell lymphoma in 20 of 22 cases, but not with multiple myelomas, T-cell lymphomas, or Reed-Sternberg cells of Hodgkin's disease. Immunoprecipitation studies revealed that FB1 recognizes the same two polypeptide chains that are recognized by L26 and is a member of the CD20 antibody cluster. FB21 was thought to recognize a sialic acid-dependent carbohydrate epitope and this was confirmed at the Fifth International Conference on Human Leukocyte Differentiation Antigens (Boston, 1993). FB21 did not react with splenic MrZ B cells and was different from the pan B markers reported previously [CD20 (L26), CD45RA (MB1), and CD74 (LN-2)]. FB21 recognizes a subset of B cells and appears to be closely related to CD75/76 antibodies. FB1 and FB21 are useful MoAbs for the diagnosis and analysis of B-cell lymphomas.