The origin and kinetics of mononuclear phagocytes can only be studied after the cells have been properly characterized. The basis for such a characterization on morphological, cytochemical, functional, and immunological grounds has been discussed. The production and kinetics of mononuclear phagocytes during the normal steady state have been described and compared with the effect of an acute inflammatory stimulus on these parameters. The influence of hydrocortisone and azathioprine was also studied in this connection. The increased production and monocytosis seen during an acute inflammatory response have been shown to be regulated by a humoral factor. The concept of the mononuclear phagocyte system was also discussed and an outline of the participation of mononuclear phagocytes in pathological processes put forward.