Much of our understanding of the functions and properties of macro-phages has relied upon studies of these cells in tissue culture. In general peritoneal cells have served as the usual source of macrophages. This population is readily accessible and easily cultured, and has provided abundant information concerning the phagocytic and immunologic properties of macrophages. 1-In the intact animal, however, peritoneal macrophages make up only a small proportion of the total body macro-phages; the majority are fixed cells of the reticuloendothelial system (RES), mainly in the liver and spleen. Since tissue cultures of macro-phages from these origins have not previously been available for comparative study, it is not known whether peritoneal macrophages are representative of macrophages in general or whether they are a selected population. Clearly, the availability of methods by which macrophages from additional sources may be cultured would be useful for further studies on macrophage activity. In this report, therefore, results of experiments are presented in which macrophages from spleen, bone marrow, liver, lung and peripheral blood in addition to peritoneum were obtained from mice and cultivated in vitro.