The urines of 33 patients with various renal diseases and proteinuria from 0.5 up to 4.7 g/l were examined by isoelectric focusing in flat polyacrylamide gel with pH range of 3.5-10. According to the type of the individual proteins excreted in the urine, identified by immunoelectrophoresis, the following types of proteinuria were differentiated: physiological--4 patients, moderately selective--26 patients, and feebly selective--3 patients. The isoelectric focusing of the urine revealed that the urine proteins were distributed in pH range 3.5-6.3. The lower the proteinuria, selectivity--the more proteins are found by isofocusing and the more they engage the cathode part of the pH-gradient. 22 urines can be examined simultaneously. The isofocusing allows the differentiation of the proteinuria and yields results well comparable with those of the immunoelectrophoresis.