The aim of this study was to produce monoclonal antibodies to cultured human glomerular mesangial cells in order to obtain specific markers for these cells and to aid the study of their function. Using standard monoclonal antibody techniques, 29 hybridomas producing antibodies directed to cultured mesangial cells were obtained. Most of these antibodies were not reactive with normal or neoplastic haematopoietic cell lines by flow cytometry. Fourteen of the 29 culture supernatants bound to various components of normal human kidney sections stained by the alkaline phosphatase/anti-alkaline phosphatase (APAAP) method. Ten of these supernatants reacted with components within the glomerulus, with six binding to the mesangium. These studies suggest that (1) mesangial cells in culture may show significant de-differentiation, because most supernatants which reacted with mesangial cells in culture did not do so in tissue sections; (2) antibodies reactive with haematopoietic cells may not detect the majority of immunogenic surface antigens on cells in tissues; and (3) some of the antibodies which we have produced may prove to be useful markers for mesangial cells in glomerular disease.