On 112 patients with bioptically ascertained chronic proliferative glomerulonephritis, 94 with pyelonephritis and 23 patients with kidney transplantation altogether 1,050 times the fibrin fission products in the urine were estimated by the passive haemagglutination after Merskey. It was the aim of the investigation to test the diagnostic evidence described in literature concerning the floridity diagnostics in glomerulonephritis and the recognition of rejection in kidney transplantation as to its reproducibility. In comparison to the latent glomerulonephritis (0.3 microgram/ml) the florid glomerulonephritis (12.3 microgram/ml) as well as the acute pyelonephritis (9.2 microgram/ml) in comparison to the chronic pyelonephritis (1.3 microgram/ml) has significantly higher values. On account of the numerous "falsely positive" and "falsely negative" values in contrast to the data of other authors an activity diagnostics is not possible. Only in the glomerulonephritis with nephrotic syndrome a prognostic use is to be expected: Patients with steroid-sensitive nephrotic syndrome do not secrete any fibrin fission products in the urine and patients with steroid-resistant nephrotic syndrome secrete them in a large number. We could confirm that an increase of the fibrin fission products in the urine after kidney transplantation refers to an acute rejection crisis. Since 10 of 27 rejections were fibrin fission product-negative, in the lacking fibrin fission products in the urine a rejection is not be excluded, by which the diagnostic value is restricted.