Preliminary injection of 2 mg/kg diazepam abolishes the pyrogenic reaction and increases the survival rate of rabbits after intravenous injection of cat blood. A study of the physicochemical properties of the blood by measurement of the ionization equilibrium during thermal denaturation showed that injection of heterologous blood into rabbits and suspending rabbit''s erythrocytes in cat''s plasma in vitro lead to a sharp decrease in the rate of proton binding by the plasma protein in the presence of the foreign erythrocytes. Preliminary injection of diazepam to the rabbits and addition of diazepam to the cat''s plasma in vitro prevent these changes and enable the blood to retain its native properties. Addition of diazepam in a concentration of about 10^–4 M to heterologous plasma considerably reduces erythrocyte agglutination.