The structure of protein A-coated colloidal gold particles, and of macrophage cell-surface receptors conjugated with immunogold particles, was studied using an ultrahigh-resolution scanning electron microscope. Protein A, when conjugated with 15-nm gold, formed a coat completely surrounding the particle. Particles conjugated with both protein A and immunoglobulin G (IgG) were similar, but with additional protrusions formed by the IgG. IgG molecules directly bound to gold were resolved sometimes as complexes of three units, sometimes as more filamentous, V-shaped structures. On the cell surface of macrophage reacted with a monoclonal antibody to Mac-1 antigen (the murine C3bi receptor) followed by protein A-gold, gold particles were seen to be linked via the IgG to the receptor, visualized as a round granule.