Use has been made of the radioactive properties of colloidal thorium dioxide, intravenously injected into rabbits, to study the functional capacity of the reticulo-endothelial cells for phagocytosis. A mathematical analysis of the problem is presented which indicates that when the system is not overloaded, the foreign material should be removed from the blood according to the relation . Here a≫b and n is the number of particles remaining in the blood at any time t. Experiments on 13 rabbits substantiate this prediction. With a standard injection of 1 cc of colloidal thorium dioxide per kilogram of body weight, n decreases according to three distinct exponential relations. The system is apparently overloaded initially, but after n decreases to about one-sixth its initial value (at t=0), the fraction of the amount of material present that is removed in unit time increases and the rest of the curve follows the predicted relation. The fact that the system is initially overloaded was checked by the use of smaller amounts of thorium. In all cases more than 99 percent of the thorium was removed from the blood stream within 6 or 8 hours after the injection. The technique reported here should serve as a basis for testing many theories of phagocytosis now largely based on deduction.