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STIMULATION of bovine adrenal chromaffin cells results in a rise in the concentration of cytosolic calcium^1-3 which triggers the release of catecholamines by exocytosis. Several cytosolic proteins that bind to secretory granule membranes in a calcium-dependent manner have been implicated in exocytosis^4-12 and some belong to a family of calcium-binding proteins, the annexins⁶. One of these, calpactin, is a tetramer consisting of two heavy and two light chains (relative molecular masses 36,000 and 10,000 respectively)^7,8 and can aggregate^10,11 and fuse membranes in vitro in the presence of arachidonic acid¹⁰. Calpactin is found at the cell periphery^13,14 and is phosphorylated when chromaffin cells are stimulated¹⁵. We show here that both calpactin and calpactin heavy chain (p36) reconstitute secretion in permeabilized chromaffin cells in which secretion has been reduced as a result of leakage of cellular …