Changes of peroxidase (PO) activity in blood monocytes, untreated or stimulated peritoneal macrophages and subcutaneous exudate macrophages of rabbits were investigated with the glucose-oxidase method in the culture experiments of these cells after their in vitro or in vivo adherence to the surface of foreign bodies. 1) PO activity was demonstrated in the nuclear envelope (NE) and rough endoplasmic reticulum (rER) in 75% of the untreated peritoneal macrophages. 2) Blood monocytes, most of the stimulated peritoneal macrophages, and subcutaneous exudate macrophages showed no PO positivity. 3) In the in vitro culture of blood monocytes and stimulated peritoneal macrophages, there appeared from 2 hours cells showing PO positivity in NE and rER. 4) Although blood monocytes, stimulated peritoneal macrophages, and subcutaneous exudate macrophages after their in vivo surface adherence showed no PO positivity, it appeared in these cells when further cultured in vitro. From the fact that such a positive change of PO activity in these cultured cells is not only a transient phenomenon but also in vitro cultural one, we cannot subscribe to the view that blood monocyte is the precursor of resident macrophages.