The concentrations of fibrin degradation products (FDP) in the urine were determined by the passive hemagglutination test in 115 patients with biopsy-proven chronic proliferating glomerulonephritis (GN), 93 patients with urinary tract infection (UTI) and 23 patients who received kidney transplants. The active GN values (12.3 micrograms/ml) are significantly higher than those for latent GN (0.3 micrograms/ml). Those for acute UTI (9.2 micrograms/ml) are significantly higher than for chronic UTI (1.3 micrograms/ml). In contrast to the reports published by others, the numerous 'false-positive' and 'false-negative' values make diagnosis of the activity questionable. Prognostic value can be expected in GN with the nephrotic syndrome (NS): patients with steroid-sensitive NS excrete FDP and patients with steroid-resistant NS excrete larger quantities of FDP. We have confirmed that a rise in the urinary FDP level in transplantation is indicative of rejection. However, since 10 of 27 rejections were FDP-negative, the absence of FDP in the urine does not preclude rejection.