My studies have investigated the possible involvement of the plasminogen activator/plasmin system in glomerular physiology and pathology. Urokinase plasminogen activator and its cellular receptor are not found immunocytochemically within the normal or diseased glomerulus in vivo. Plasminogen activator inhibitor-1, an inhibitor of urokinase plasminogen activator was, however, identified in crescents from cases of crescentic glomerulonephritis, but not within the glomerular tuft proper. Culture of human glomerular cells initially revealed urokinase plasminogen activator and plasminogen activator-1 proteins within the supernatant of epithelial cells and mesangial cells, as measured by ELISA. However, immunocytochemical characterisation of each of the cell cultures showed that there was contamination of some of the mesangial cell cultures by epithelial cells. Pure cultures of both types of cell produced plasminogen activator inhibitor-1. However, measurement of urokinase plasminogen activator activity by zymography confirmed that this molecule was not present within supernatants obtained from pure mesangial cell cultures. Furthermore, the use of combined non-isotopic in situ hybridisation and immunocytochemistry allowed identification of urokinase plasminogen activator mRNA only within human cultured glomerular epithelial cells and not within mesangial cells. This finding proved that contaminating epithelial cells were responsible for urokinase plasminogen activator production in cultures thought to be made up of pure mesangial cells. Thus there is excellent evidence for synthesis of this molecule only by epithelial cells. Non-isotopic and radioactive in situ hybridisation were unsuccessful in identifying urokinase plasminogen activator within human kidney sections and the difficulties involved in the methodology of this technique, and the implications of the culture cell work for the role of the plasminogen activator/plasmin system in the glomerulus, are discussed.