By means of in vitro phagocytosis it was demonstrated that macrophage-like blood cells of an invertebrate (amoebocytes of the pond snail Lymnaea stagnalis) possess cytophilic receptors for foreignness. These receptors are lectin molecules which are reversibly bound to the surface of the cells. Phagocytosis is dependent, beyond recognition of foreignness, on temperature and pH, and on the feeding condition of the snails.