The binding of 125iodine labelled human IgG to allogenic erythrocytes was studied at various ratios of free IgG per cell. At low concentrations of free IgG a high affinity binding was measured, which is with respect to its apparent association constant (2.8 x 10(13) mol-1) most likely a specific receptor binding and might indicate aged cells of those destined for elimination. At higher concentrations of free IgG the erythrocytes were coated by IgG in a non-saturable manner as reported by Grob et al. (Immunology 13, 189 (1967)). The erythrocyte-bound labelled IgG, which remained in the range from 15 to 400 molecules IgG per cell after intensive washings could not be chased by unlabelled IgG. This cell-bound IgG (high affinity binding) was increased 7-fold after complete ATP-depletion of erythrocytes and was 4 times higher after erythrocyte storage for 42 days. It appears that it is not the number of bound IgG molecules alone which is important for erythrocyte recognition by macrophages but also the arrangement of IgG molecules bound to membrane polypeptides.