Ingestion by mononuclear cells in bone marrow, peripheral blood and the peritoneal cavity of red cells treated with antibodies and various chemicals was studied in an attempt to characterize these cell lines during development and to clarify the interrelationship among them. Bone marrow mononuclear phagocytes became intensively phagocytic during the second day of culture, indicating that the maturation time from progenitor cells to macrophages was about 48 hr. Blood monocytes exhibited very active erythrophagocytic activity after 24 to 48 hr cell culture. Uptake of sensitized red cells by both the cell lines was markedly inhibited by IgG irrespective of time periods of cell culture. Majority of peritoneal lymphocyte-like cells (LLC) separated from other forms of mononuclear cells were shown to become capable of erythrophagocytosis with maturation time to macrophages between 24 to 48 hrs. Under non-stimulated steady state, uptake of sensitized red cells by peritoneal macrophages was far less sensitive to IgG inhibition than that by bone marrow mononuclear phagocytes and blood monocytes, suggesting that IgG receptor does not play main role for the erythrophagocytosis by peritoneal macrophages. Peritoneal cells collected under inflammatory stimulus exhibited a character similar to bone marrow mononuclear phagocytes and blood monocytes with regard to IgG receptor. It was suggested that under steady state peritoneal macrophages arise and mature from local proliferative pool with some different phagocytic mechanism from that of bone marrow mononuclear phagocytes.