N-ethylmaleimide-sensitive fusion protein (NSF) is a component of intracellular transport reactions. In order to understand the role of NSF during the fusion of endocytic transport vesicles with the endosome, we have investigated the binding of NSF to purified clathrin-coated vesicle components. First, we have examined whether detergent-solubilized coated vesicle membranes will support formation of NSF-containing 'fusion complexes'. Our results show that these membranes are substantially enriched in components capable of driving formation of these complexes, when compared with membranes from other sources. Secondly, we have analysed coated vesicle preparations for their NSF content. Coated vesicle preparations contain significant amounts of NSF. This was shown to be associated with coated vesicles rather than contaminating membranes by a number of criteria, and was found to be bound in an ATP-independent manner. These findings are discussed in the light of current models for vesicle fusion.