A murine monoclonal antibody (designated R4/23) which reacts strongly with human dendritic reticulum cells (DRC) is described. Immunoperoxidase staining of tissue cryostat sections revealed that this antibody reacts strongly with DRC in lymphoid follicles (both primary and secondary), and also weakly with marginal zone splenic B cells and with some peripheral follicular mantle B lymphocytes in lymph node cortical follicles. The value of antibody R4/23 is that it allows the distribution of DRC in reactive and neoplastic lymphoid tissue to be clearly delineated. Of particular interest is the fact that all cases of follicular lymphoma of germinal centre cell origin are consistently accompanied by a proliferation of DRC, even when the neoplasm is present in non-lymphoid tissue--for example, in the kidney. In contrast, DRC in B cell lymphomas of non-germinal centre origin are partially or totally obliterated.