The authors propose a new method for the interpretation of the measurement data obtained during radionuclide investigations of the function of the reticuloendothelial system (RES). This method is upon the concept of a considerable effect of the blood opsonizing factor on the transport of i.v. injected radioactive colloid in the body. A non-linear compartmental model was developed that describes the binding of colloid particles with opsonins in the vascular bed followed by absorption of the particles of the colloid-opsonin complex by RES cells. The identification of the parameters of the non-linear model by the results of dynamic scintigraphy of the heart and liver makes it possible to explain some phenomena in the measurement data and to assess the relative amount of opsonins in the blood and RES phagocytic activity as a whole and its constituents. It was shown in particular in the studies on patients with hepatocirrhosis that the amount of opsonins in the blood remains almost the same as compared to the normal one whereas the phagocytic activity of RES on the whole, hepatic stellate cells and RES extrahepatic cells decreases significantly.