Immunocytochemical studies on both the light-microscopic and electron-microscopic levels proved that peritoneal resident macrophages derive from a precursor cell in the bone marrow. Evidence that this precursor cell differs from that of the monocyte is described. Progenitor cells of peritoneal resident macrophages were found in the peritoneal milky spots. Peritoneal resident macrophages show local proliferation, as determined by 3H-thymidine labelling and electron-microscopic autoradiography. On the basis of these findings it is postulated that peritoneal resident macrophages derive from locally proliferating progenitor cells which themselves arise from specific stem cells in the bone marrow, differing from the stem cells of the monocyte/granulocyte series.