Chromaffin granules of the adrenal medulla are surrounded by a single unit membrane. So far no special morphological characteristics of these membranes have been described. However, biochemical analyses have revealed the special properties of these membranes. The lipids are characterized by a high content of lysolecithin. It has been suggested that this specifically localized phospholipid is essential for the secretion of catecholamines, which involves membrane fusion. The proteins of the granule membrane have also been investigated. Two major components appear to be specific for chromaffin granules of several species. Three enzymes, namely an Mg 2+ -activated ATPase, dopamine β-hydroxylase and cytochrome b-559, are also known to be present in the granule membranes. The membranes of these organelles have no common structural backbone with microsomal membranes.