Fibrin formation plays an important role in glomerular injury. We therefore examined the procoagulant signal produced by cultured rat mesangial cells. Actively growing mesangial cells produced procoagulant activity (PCA) that was present in intact cells (surface-associated), was inhibitable by cyclohexamide and which, by clotting assay, had the characteristics of tissue factor. This PCA decreased with incubation of cells in serum-deprived medium. Incubation with bacterial lipopolysaccharide (LPS) and tumor necrosis factor (TNF alpha) induced increased detectable tissue factor by mesangial cells within two hours which was maximal by four hours. We conclude that quiescent mesangial cells produce a small amount of tissue factor-like procoagulant activity, and that this PCA can be stimulated by incubation with TNF alpha, LPS or when cells are actively growing in high serum medium. Therefore mesangial cells have the capability of contributing to fibrin formation during inflammatory glomerular injury or sepsis.