Previous studies have demonstrated that GTPgS and rab5:Q79L, a constitutively active rab5 mutant, stimulate endosome fusion in in vitro assays [Cell Reg. 1:113-124 (1989)]. Additionally, electron microscopy has shown that under these conditions, endosomes form incompletely fused couplets [Cell Reg. 1:113-124 (1989); Arch. Biochem. Biophys. 326:64-72 (1996)]. CHO and BHK cells that overexpress rab5:Q79L develop ‘giant’ cytoplasmic vesicles that exhibit many characteristics of early endosomes including immunoreactivity for rab 5 and transferrin receptor (data not shown). The formation of these vesicles has been analyzed by time-lapse video microscopy which shows the enlarged endosomes arise primarily by fusion of smaller vesicles. These endosome fusion events occur by two distinct mechanisms that differ in temporal and spatial characteristics of the membrane merger process and we refer to these processes as “explosive” versus “bridge” fusion. “Explosive” fusion is a rapid process in which the initial opening that forms between contacting vesicles expands rapidly and permits an abrupt, complete coalescence of membranes (Fig. 1).