Immunoelectron microscopy of vesicles isolated on magnetic beads coated with antibodies to CR1 peptides. (A) Vesicles stained with a mixture of MoAbs 3D9 and C543 to the extracellular domains of CR1. (B) Vesicles stained with antihuman albumin MoAb. (C) Vesicles stained with control nonimmune antibody MOPC 21. Arrows show vesicles specifically bound to beads bearing antibodies to CR1 tail peptides. (A) and (B) show staining with 5-nm gold-conjugated goat antimouse IgG. (C) fails to show staining with the same gold-conjugated goat antimouse IgG. Bars = 0.1 μm.