Abstract. The molecular details of the final step in the process of regulated exoc'ytosis, the fusion of the membrane of the.secretory granule with the plasma membrane, are at present obscure. As a first step in an investigation of this membrane fusion event, we have developed a cell-free assay for the interaction between pancreatic zymogen granules and plasma membranes. We show here that plasma membranes are able to trigger the release of the granule contents, and that this effect is specific to pancreatic membranes, involves membrane fusion, requires membrane proteins, and is stimulated by activators of G-proteins but not by Ca 2+. The assay is simple, reliable, and rapid, and should permit the identification of proteins that are involved in the exocytotic fusion event.ultIr~G the process of regulated exocytosis, cell activation generates intracellular signals that trigger fusion between the membranes of secretory vesicles and the plasma membrane, leading to the release of the vesicle con-