The adherence, phagocytic activity and buoyant density of mouse peritoneal exudate colony forming units (CFU‐PE) were investigated. There was a significant enrichment in the proportion of CFU‐PE in the adherent cells population, defined as cells adhering to a plastic surface within 30 minutes of incubation. The phagocytic activity of CFU‐PE was studied by incubating exudate cells with iron particles for 45 minutes. The cells were then separated into phagocytic and non‐phagocytic cell fractions by passing the incubation mixture through a magnetic field. A significant enrichment of CFU‐PE was seen in the phagocytic cell fraction. When exudate cells were fractionated in a Ficoll discontinuous density gradient, more than 88% of CFU‐PE were recovered at the 16/18% and 18/20% interfaces. It is concluded that CFU‐PE are adherent cells, have strong phagocytic activity and have a buoyant density between 1.0562 and 1.0703. When bone marrow cells were studied by these techniques, the committed stem cells for both granulocytes and macrophages (CFU‐C) were enriched in both non‐adherent cell and non‐phagocytic cells populations. In the Ficoll density gradient, CFU‐C banded at a heavier density region than CFU‐PE.