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These studies were undertaken with the aim of detecting possible differences between immunologic binding of IgG antibodies to erythrocyte membranes and the well documented nonimmunologic adherence to erythrocytes of large numbers of IgG molecules. In the experiments described D+ (Rho) erythrocyte ghosts were incubated with either 125I labelled anti-D or 125I IgG and then thoroughly washed. The ghosts which contained either immunologically bound anti-D or adherent IgG were then passed through a French press which converted the ghosts into micro-vesicles composed of membrane material. This procedure resulted in the selective removal of nonimmunologically adherent IgG from the micro- vesicles while leaving most of the antibody still firmly bound to the micro-vesicles. A series of control studies revealed that this observation was not limited to a single preparation of anti-D or IgG, and was not related to the number of IgG molecules attached to the erythrocyte membrane. Selective methods of chemical elution did not distinguish between the two forms of IgG attachment nor did another physical manipulation i.e. freezing and thawing. Separation of the sheared micro-vesicles into distinct classes indicated that antibody binding and nonimmunologic adherence of IgG molecules occurred equally to each of the different vesicle classes. These observations indicated that antibody binding and nonimmunologic adherence of IgG differ with regard to their physical characteristics and this difference may be a reflection of the inability of nonimmunologically adherent molecules to react with the Coombs' reagent. [ABSTRACT FROM AUTHOR]