One of the main objectives of immunopotentiators is their ability to enhance or to restore natural anti-infectious resistance of normal or immunocompromised hosts. Numerous experimental resistance models have been used in screening such potentiators. However, once such a substance has been selected, its potential and practical use will directly depend on the knowledge of the underlying resistance mechanisms induced, defining its cellular or molecular targets. Phagocytosis-polymorphonuclear or monocyte-macrophage dependent--is the most frequently described variable which have been analysed among the various potentially active mechanisms of non-specific resistance. Various in vivo and in vitro tests might be used and the target of several immunostimulants among the different steps of phagocytosis are described. However, numerous intrinsic limitations are associated with such tests and news models or tests are presented and discussed in order to gain more insights into the evaluation of active substances.