Ultrathin frozen sections of human platelets isolated as in Fig 1 and immunolabeled with monoclonal antibodies to clathrin followed by goat anti–mouse IgG bridge antibody and 5-nm immunogold particles. After blocking in 1% glutaraldehyde in PBS for 10 minutes, the sections were immunolabeled with polyclonal antibodies to Fyn, followed by goat anti–rabbit IgG bridge antibody and 10-nm immunogold particles. (a) and (b) Label for Fyn and clathrin are associated (small and large arrows, respectively). Similar results were obtained with protein A–gold or IgG-gold probes. In no double-label experiment was the amount of 10-nm gold probe the same as in a single label for the same primary antibody, probably due to steric hindrance of the gold particles. G, alpha granule. Original magnification × 70,000.