A collection of new and established FDC-reactive MoAb has been used in an immunohistological study designed to throw light on (a) the nature of FDC (which are of unknown lineage) as judged by their sharing of antigens with other cell types and (b) the reason for the strong expression of some B cell antigens on FDC. The MoAb were tested on: (1) sections of tonsil, (2) sections of lymph nodes from four cases of non-Hodgkin's lymphoma, (3) peripheral blood cells and (4) cells of cultured haemopoietic cell lines. Only one of the ten new MoAb bound to FDC and no other component of the tissues screened. It resembled R4/23, a MoAb known to be specific for FDC. The other nine antibodies showed a range of cross-reactivity patterns involving one or more of the following: monocytes, macrophages, platelets, epithelium, endothelium and connective tissue fibres. Some of the MoAb reacted with B lymphocytes and cells of B lymphoblastoid lines but none showed the restricted FDC-staining pattern associated with MoAb which detect the CD23, P45 antigen. The findings are discussed in terms of the intrinsic or extrinsic nature of the antigens detected.