Immunoelectron microscopic localization of CR1 in resting RBL. Cryosections were stained with a mixture of two monoclonal antibodies directed against CR1 (3D9 and C543) followed by rabbit antimouse IgG conjugated to 5 nm gold. CR1 is shown on the cell surface, in small vesicles (small vertical arrows), and in multivesicular bodies (large horizontal arrows). The inset shows RBL-CR1 cells that were incubated with BSA conjugated to 20 nm gold for 30 minutes at 37°C before preparation for cryosectioning. CR1 was labeled with 5 nm gold as described above. Colocalization of the endocytic marker BSA-gold (large particles) and CR1 (small arrow heads) was observed in multivesicular bodies. Bar = 0.1 μm.