Phagocytosis in leukocytes has been shown to be accompanied by the stimulation of a number of metabolic functions. The process is dependent for the energy required on some of these changes, while others are simply concomitants of particle ingestion [1, 2, 3, 4]. There are reports in the literature to the effect that various substances, such as endotoxin for example, cause metabolic changes in leukocytes that are similar to those seen during phagocytosis [5, 6]. The present study has extended these observations and has attempted to utilize the agents that stimulate the cells as a possible “probe” of the cell membrane and as a means for exploring the events that occur when phagocytosis is initiated, i.e., the “trigger” events. These studies have involved several cell types (polymorphonuclear leukocytes and monocytes from the peritoneal cavities of guinea pigs and rabbits, guinea pig alveolar macrophages and mouse peritoneal macrophages). They have been pursued by following metabolic changes and by making observations with the electron microscope. The guinea pig neutrophilic granulocytes and monocytes, on which the largest amount of metabolic data are already available for the phagocytic event, have been subjected to the closest examination with respect to the action of agents at concentrations around 25µg./ml.