Human blood monocytes when cultured on hydrophobic Teflon membranes differentiate into mature macrophages. The expression of transferrin receptors was monitored by monoclonal antibody (OKT9) binding as detected by immunoperoxidase staining. Whereas monocytes were negative, an increasing percentage of macrophages, starting from day 2 in culture, labelled with the antitransferrin receptor antibody as these cells undergo differentiation. After completion of maturation more than 90% of macrophages expressed transferrin receptors. While 90-95% of macrophages from broncho-alveolar lavage fluids labelled with the OKT9 antibody, only a minor portion of macrophages obtained from peritoneal and pleural cavities did so. In parallel, intracellular ferritin in cells of the monocyte-macrophage lineage increased from 10 ng/10(6) cells to 350-1,500 ng/10(6) cells during maturation in vitro. Alveolar macrophages proved to have the highest ferritin content which ranged from 355-8,400 ng/10(6). The results may indicate that iron uptake and storage is a function of cells at late stages of macrophage maturation and that the occurrence of surface receptors for transferrin can be regarded as differentiation dependent marker.