Rabbit blood monocytes, which contain no cytochemically demonstrable peroxidase, develop peroxidatic activity in the RER and perinuclear cisternae within 2 h after adherence to serum- or fibrin-coated surfaces. A similar reactivity appears in surface-adherent human and rat blood monocytes. In both localization and characteristics, this enzyme reactivity in monocytes resembles that normally seen in the resident peritoneal macrophages of the rabbit, as well as in several types of tissue macrophages in other species. Thus this observation supports the concept, presently based on the kinetic data of other investigators, that blood monocytes are the precursors of such cells. Moreover, the appearance of new enzyme activity after adherence may reflect alterations in cellular metabolism resulting from plasma membrane:surface interactions.