At least two distinct ATPases, NSF and p97, are known to be involved in the heterotypic fusion of transport vesicles with their target membranes and the homotypic fusion of membrane compartments. The NSF-mediated fusion pathway is the best characterized, many of the components having been identified and their functions analysed. In contrast, none of the accessory proteins for the p97-mediated fusion pathway has been identified. Now we have identified the first such component, a protein of relative molecular mass 47,000 (p47), which forms a tight, stoichiometric complex with cytosolic p97 (one trimer of p47 per hexamer of p97). It is essential for the p97-mediated regrowth of Golgi cisternae from mitotic Golgi fragments, a process restricted to animal cells. As a homologue of p47 exists in budding yeast, this indicates that it might also be involved in other membrane fusion reactions catalysed by p97, such as karyogamy.