Human blood-born monocytes have been cultivated on the hydrophobic side of Teflon foils (fluorinated ethylene propylene copolymer) using human pooled AB-serum as essential growth factor. At any stage of culture these in vitro maturing macrophages can easily be detached from the Teflon membrane and subjected to further experimentation. Once established, primary macrophage cultures can be maintained in medium containing 10% FCS for up to 3 months. Lysozyme secretion increased more than 10-fold during the sequential process of monocyte transformation into macrophages and correlates with cell number and stage of maturation. The ability to inhibit growth of human permanent tumor cell lines also developed during macrophage maturation. Studies on the cells of 22 healthy donors revealed a reproducible activity of mature macrophages against K562 and MOLT4 tumor cells. Our system will facilitate investigations on various aspects of human macrophage differentiation and function.