I n vitro erythrocyte membrane internalization, resulting in the formation of membrane-lined vacuoles, can be quantified by a radioisotopic method. A complex of ³⁷Co-labeled vitamin B₁₂ and its plasma protein binders is first adsorbed to the cell surface, and after vacuoles are formed, the noninternalized label is removed by washing and trypsin treatment. The residual radioactivity represents trapped label and can be used to measure the extent of membrane internalization.Using this method, it was found that in addition to primaquine, a group of membrane-active drugs, specifically hydrocortisone, vinblastine, and chlorpromazine can induce membrane internalization in erythrocytes. This is a metabolic process dependent on drug concentration, temperature, and pH. Vacuole formation by all agents tested can be blocked by prior depletion of endogenous substrates or by poisoning the erythrocytes with sodium fluoride and sulfhydryl blocking agents. This phenomenon resembles in some respects the previously reported membrane internalization of energized erythrocyte ghosts. It is suggested that membrane internalization is dependent on an ATP-energized state and is influenced by the balance between the concentrations of magnesium and calcium in the membrane. This study provides a basis for proposing a unifying concept of the action of some membrane-active drugs, and for considering the role of erythrocyte membrane internalization in pathophysiologic events.