Mesangial cells are considered to have an important role during the development of some forms of glomerulonephritis in which mesangial cell proliferation and extracellular matrix expansion is prominent. The aim of this study was to define changes in extracellular matrix components derived from mesangial cells after in vitro stimulation with cytokines, and, using monoclonal antibodies developed against such extracellular matrix components, to localise and define antigens in non-stimulated and stimulated mesangial cells and in normal and diseased kidney sections. Using standard methods such as the APAAP staining technique, dot-blot assays and SDS gels with the mesangial cells, matrix and supernatants, it was shown that mesangial cells stimulated with rIL-1 or PMA expressed novel antigens compared to resting cells: One particular protein was expressed by mesangial cells only after stimulation. It was shown, therefore, that cytokines and PMA can alter extracellular matrix components of mesangial cells in vitro. Each of the cytokines studied resulted in separate patterns of extracellular matrix component synthesis for the different adherent cell lines, but, in general, cytokines increased the amount of fibronectin, proteoglycans and vitronectin produced, while decreasing or not affecting the concentration of collagen IV. In a further study, ninety one hybridomas against extracellular matrix of stimulated mesangial cells with PMA or rIL-1 were produced from four fusions. Fourteen monoclonal antibodies were chosen for cloning by limiting dilution technique. One of these, monoclonal 84, reacted only with stimulated mesangial cells and did not react with any of the other adherent or haemopoeitic cell lines studied. It also reacted with diseased kidney sections but not with normal kidney.