A powerful procoagulant is normally excreted in large amounts in the urine of man, rats and rabbits. The secretion is not altered in a variety of diseases including hemorrhagic diathesis and thromboembolism. In parenchymatous kidney diseases, however, there is a marked reduction of procoagulant excretion. The procoagulant excretion is negatively correlated with BUN, blood creatinine and degree of proteinuria. With marked proteinuria in human or experimental renal disease, the procoagulant content per ml urine and the 24 h excretion drop to very low or zero values. In man, complete recovery of procoagulant excretion is obtained after homotransplantation of a well functioning kidney.