This chapter discusses transcytotic vesicle fusion with plasma membrane. Cell-free assays provide superior experimental systems in which to identify and characterize the components and the molecular mechanisms that underlie membrane transport events. General parameters and specific components required for protein transport have been defined in a number of reconstituted systems. Such in vitro assays have been developed to analyze traffic from the endoplasmic reticulum (ER) to the Golgi between Golgi cisternae, from the trans-Golgi network (TGN) to the plasma membrane (PM), from the endosome to the TGN from the PM to an endosomal compartment, s and among the endosomal compartments. Within the early endosome, the proteins are sorted and packaged into specialized carriers, the transcytotic vesicles, which are then translocated across the cell and specifically fuse with the opposite PM. © 1992, Elsevier Inc. All rights reserved.