A new mathematical approach to the calculation of the kinetics of macrophages in a tissue compartment is presented. This approach, which takes into account the influx of monocytes into the compartment, the local division of mononuclear phagocytes, and the efflux of macrophages from the compartment, was applied to data on the pulmonary macrophages of mice in the normal steady state. The results show that at least 70% of the pulmonary macrophage population is supplied by monocyte influx and at most 30% by local division of immature mononuclear phagocytes originating from the bone marrow. The calculated turnover time of pulmonary macrophages is about 6 days, and the turnover amounts to 14.6 X 10(3) macrophages/hr.