Heme-microbeads attached themselves to the surface of viable K562 cells in a manner inhibitable by free hemin, indicating heme-receptor interaction. The microbeads were at first evenly distributed, but after prolonged incubation at 37 degrees C they formed a cap on one pole of the cells indicating clustering of the membrane heme receptors. Membrane proteins were labeled by culturing the cells in the presence of 35S-methionine and were then solubilized with Triton X-114. The hydrophobic proteins contained about 20% of the total bound label. The solubilized membrane proteins were subsequently adsorbed to a heme-Sepharose affinity gel. According to SDS-electrophoresis and subsequent autoradiography, the immobilized heme captures two proteins or a protein with two polypeptides of 20,000 and 32,000 daltons. The larger of these was only weakly labeled with 35S. The same two bands were observed if the cell surface proteins were labeled with 125I by the lactoperoxidase method and the subsequently solubilized membrane proteins were isolated with heme-Sepharose.