The function of the reticulo-endothelial system in mice bearing Lewis lung carcinomas has been measured by(1) the rate of clearance of carbon particles from the circulation in vivo and calculation of the phagocytic index K; (2) chemotactic locomotion of macrophages in vitro in the presence or absence of serum or tumour supernate. The ability of the bone marrow to develop macrophage colonies in vitro in the presence or absence of sera from tumour-bearing mice has also been tested. A clear depression of macrophage locomotion and macrophage colony formation in vitro was found in the presence of sera or tumour supernates from tumour-bearing mice as early as 24 to 72 h after tumour inoculation. Similarly, tumour-bearing mice showed marked depression of carbon clearance in tests repeated throughout the first 72 h after tumour inoculation. This early depression of macrophage function may be an important step in allowing escape of tumour cells from host resistance.