C3b, the major cleavage fragment of the third component of complement (C3), has been demonstrated to bind to a specific receptor on various mammalian cells1. Tissue-bound C3b receptors have also been demonstrated, most conclusively in renal glomeruli2. On interaction with C3b, C3b receptors are thought to initiate cellular functions such as phagocytosis and to determine the fate of complement-fixing soluble and particulate immune complexes. We report here the isolation by affinity chromatography of a macrophage glycoprotein with an apparent molecular weight (MW) of ∼64,000 that has properties expected of the C3b receptor. It is a cell-surface macromolecule (labelled with 125I and lactoperoxidase) which, in its isolated state, retains the ability to bind both C3 and C3b.