The plasmin protease system may have a role in maintaining the patency of renal tubules and in regulating matrix degradation within the glomerulus. Urokinase-plasminogen activator (u-PA) is a serine protease which plays an important part in the regulation of plasmin production from plasminogen. The synthesis of u-PA by cultured human glomerular cells, in particular mesangial cells, is controversial. The present study describes the presence of u-PA in supernatants of pure cultures of human glomerular epithelial cells (EC), cocultures of EC and human mesangial cells (MC) and whole glomeruli, but not within pure cultures of MC. To confirm the synthesis of u-PA mRNA in glomerular EC, cocultures of EC and MC were tested by in situ hybridization with u-PA antisense and sense digoxigenin-labeled RNA probes. Cytoplasmic localization of u-PA mRNA was demonstrated only in the EC, thus confirming the absence of synthesis of u-PA by human mesangial cells in culture.