We have estimated the macrophage content of different tissues of the normal adult mouse using F4/80, a highly specific antigen marker for mature mouse macrophages. An absorption indirect binding assay was used to quantitate F4/80 antigen against a calibration standard made from the J774.2 macrophage-like cell line. The richest sources of tissue F4/80 antigen were found to be bone marrow, spleen, cervical and mesenteric lymph nodes, large bowel, liver, kidneys, and small bowel. The organs that have the highest total F4/80 antigen content are the liver, large bowel, small bowel, bone marrow, spleen, cervical and mesenteric lymph nodes, and kidney. We conclude that the mononuclear phagocyte system is mainly distributed in the gastrointestinal tract and liver, followed by hemopoietic and lymphoid tissues.