A description is given of the distribution of peroxidatic (PO) activity in murine monocytes of blood and peritoneal cavity, and in murine macrophages residing in the unstimulated peritoneal cavity as well as in liver, spleen, bone marrow, and small intestine. In the monocytes, PO activity is restricted to some of the cytoplasmic granules; in the tissue (or resident) macrophages present in peritoneal cavity, liver, spleen, and small intestine, the PO activity is located in the nuclear envelope and the rough endoplasmic reticulum. Macrophages in the bone marrow are PO-negative. In the spleen and bone marrow, reticulum cells show PO activity in the nuclear envelope and the RER. Transitional forms between monocytes and tissue macrophages were not observed.