EM localization of gold-conjugated 24FM in COS.TM-CR cells. After preincubation of cells with gold-complexed antibody, the cells were exposed to 37°C for 0, 10, 20, and 45 minutes (A, B, C, and D, respectively). (A) Gold particles are diffusely distributed over the cell membrane surface (arrowheads). (B and C) Clusters of particles are seen on the cell surface and in predominantly noncoated pits and vesicles. (D) At 45 minutes, particles appear in deeper vesicles and multivesicular bodies (arrow), coincidentally with particles at the cell surface.