After reduction and splitting of disulfide linkages the fibrinmonomer and fibrin of 45 patients with histologically confirmed liver cirrhosis and 38 patients with chronic renal failure (serum creatinine greater than 5 mg%) were analysed by SDS-PAA electrophoresis. Furthermore the activity of factor XIII was measured immunologically. The results indicated no polymerization of alpha-chains of fibrin while gamma-dimers were formed regularly in 71% of patients with liver cirrhosis and in 45% of patients with chronic renal failure. In liver cirrhosis and in 45% of patients with chronic renal failure. In liver cirrhosis the lack of alpha-polymerization correlated to the severity of the disease and to the decrease of factor XIII activity (no alpha-polymers formed when below 80% of normal). In renal failure this correlation was not demonstrable since in all cases the activity of factor XIII was within the normal range. After the addition of C14-labelled urea to normal plasma during clotting an incorporation of this tracer could be demonstrated by scintiscanner diamins like urea, forming in the course of renal failure, probably serve as the "wrong substrate" for the transaminidase factor XIII.