(1) We studied the permeability of erythrocytes (human, an, chicken and frog), collodion membrane_ and frog's urinary bladder, to urea and its associated substances, alcohols and glucose. (2) Hemolysis of human erythrocytes to urea, its derivates and alcohol, is due to the penetration of these substances through the erythrocytes membrane. (3) Among urea and its associated substances, it was observed that the hemolysis time of human erythrocytes was inversely proportinal to the molecular weight of these substances. (4) Chicken erythrocytes were far less permeable to urea than human erythrocytes. (5) Hemolysis time to guanidine showed no difference between human and chicken erythrocytes. (6) Frog's urinary bladder in a living condition has a high degree of semipermeability to urea. When it is injured or dead, it behaves like collodion membrane.