EM localization of gold-conjugated thrombin in COS.TM-CR cells. After preincubation of cells with gold-complexed thrombin, the cells were exposed to 37°C for 0, 10, and 45 minutes (A, B, and C, respectively). (A) Gold particles are diffusely distributed over the cell membrane surface (arrowheads). (B and C) Particles, singly and in clusters (arrow), appear in pits, vesicles, and multivesicular bodies.