With human urine a very active procoagulant is excreted which converts prothrombin into thrombin in the presence of factor V, phospholipids and calcium chloride. In kidney diseases, its excretion is considerably reduced or totally absent. A negative correlation exists between these diseases and protein excretion. Kidney transplantation results in a normalization, however showing a trend which is not always parallel with the normalization of the blood creatinine levels. During the post-transplantation period, an occasional temporary, but very clear reduction with abnormal values of the kidney function tests can be observed. It is presumed that the procoagulant excretion could represent a hitherto unexplored function of the tubuli.