To elucidate the origin and renewal kinetics of peritoneal macrophages, as a typical example of the mononuclear phagocytic system, syngeneic rats were treated with tritiated thymidine [3H]TdR and leucocytes were transferred to unlabelled recipients over a bilateral arteriovenous shunt. Labelled and unlabelled monocytes were evenly distributed in both animals as shown by autoradiography. It was ascertained that no 'autoradiographically' detectable reutilization of label occurred and that transferred cells showed undisturbed kinetics. The results imply: resident peritoneal macrophages derive from blood monocytes; peritoneal macrophages represent a homogeneous population in respect to their cellular origin; blood monocytes as a myelogenous cell line do not represent a generative end cell. They migrate into the tissue (peritoneal cavity) and differentiate into resident macrophages, undergoing on average one mitosis per cell during a period of approximately 7 days. resident peritoneal macrophages are derived 50% from blood monocytes and 50% from division in situ; and under steady-state conditions the renewal rate amounts to 0.18%/h, which yields a half-life time of 16 days and a renewal time of 23 days.