Phagocytosis tests have been carried out by many authors using different methods under different conditions. The results have been interpreted in different ways, as well sometimes with conflicting notations. In order to get to a more systematic data analysis and to separate intrinsic from methodic influences, the possibility to apply mathematical models to a phagocytosis test has been studied. In agreement with previous experiences that phagocytosis can well be represented by a mathematical treatment as Michaelis-Menten-type enzyme kinetics concerning its initial rate and by an exponential function under in vivo conditions, in vitro-phagocytosis was phenomenologically described as an analogon of an irreversible bimolecular chemical reaction. In this way, rate and capacity of phagocytosis may be quantified separately. On the basis of systematic deviations of the data from this model, modifications have been developed which could be connected with pertinent observations. The design of further experiments from preliminary results on the basis of our models is discussed.