The present communication discusses the origin of monocytes and macrophages and gives new mathematical approach to the analysis of results obtained in studies on cell kinetics. It has been shown that the great majority of the macrophages derive from circulating monocytes, whereas the remainder arise by division of immature mononuclear phagocytes in the tissues. However, these dividing cells too originate in the bone marrow and have recently arrived in the tissues. The turnover time of tissue macrophages appears to be of the order of 1 to 5 weeks, which is shorter than had previously been calculated. During an inflammatory response, monocyte production in the bone marrow is regulated by humoral factors that stimulate or inhibit the mitotic activity of promonocytes and their precursors. Whether such factors are also present during the normal steady state is not yet certain.