Infusions of heterologous serum proteins large enough to replace the entire normal catabolic loss of the corresponding autologous proteins in the recipient rabbits caused increased rates of plasma protein catabolism, an increase in the size of the plasma protein pool and normal or even slightly increased rates of plasma protein synthesis. The principal proteins in these infusions were catabolized at rates similar to those for corresponding homologous proteins. The most marked hyperproteinemias which developed were caused principally by increases in the host's own globulin and to a lesser extent by the presence of foreign protein in the circulation.