Specific IgG antibodies directed against the spin-label nitroxide group present as a lipid hapten in single-compartment lipid vesicles have been visualized by using freeze-etch electron microscopy. Individual "particles" with diameters of the order of 20 nm are identified as single IgG molecules bound to lipid hapten. No significant aggregation of these IgG molecules was observed over a period of 1 hr in a dipalmitoyl phosphatidylcholine vesicle at 22 degrees. Binding of the human complement component C1q results in the formation of large (approximately 50-100 nm) asymmetric particles having a partially resolved substructure that may arise from individual IgG molecules bound to the membranes as well as to C1q. The binding of C1q appears to result in a clustering of membrane-bound IgG molecules. Samples containing a serum factor (perhaps anti-IgG antibodies) exhibit some IgG clustering distinct from that produced by Clq.