The possibility has been demonstrated of the use of calibrated melamine-formaldehyde latex for studying phagocytosis as an object not undergoing intracellular digestion. Latex was discovered to be actively phagocytized by macrophages and to exert no toxic action on them. A study was made of the time course of changes in the color and brightness of the fluorescence of the latex phagocytized particles in macrophages intravitally fluorochrominated by acridine orange. These changes were demonstrated to be analogous to those observed previously during phagocytosis of bacteria and other objects. The data obtained demonstrate once more the transition of the fluorescent complex from lysosomes to phagosomes and the lack of the relationship of these changes with intracellular death and digestion of the phagocytized objects. The possibility has been also shown to differentiate between phagocytized particles of latex and those located outside the cells.