Studies of the secretory process during the past decades have shown that materials destined for secretion, such as protein, glycoproteins, polysaccharides, and associated minerals are synthesised, transported and released in membrane-bound compartments. In the case of protein and glycoprotein secretion for example, such compartments include rough and smooth endoplasmic reticulum (ER), the Golgi apparatus, the condensing vacuoles, and the secretory vesicles (for review see Palade, 1975). The fusion of the secretory vesicle membrane with the plasma membrane leads to the discharge of the secretory product into the extracellular lumen and to the incorporation of the vesicle membrane components into the plasma membrane, be it transitory or for extended periods of time (for discussion see Franke & Kartenbeck, 1976). The transport of the secretion product along the discontinuous pathway through the cell occurs by fusions and fissions of the membranes involved. The recognition and interaction between these membranes is limited to definite partners and apparently takes place with a high degree of specificity (Palade, 1975). Examples include the fusion of ER-derived vesicles with the Golgi apparatus and the fusion of ER-derived or Golgi-apparatus-detached secretory vesicles with the plasma membrane. Gradual changes with respect to enzyme activities, lipid, carbohydrate and polypeptide composition, and the morphology of the specific membranes or secretory products, as well as the different turnover rates of the membrane polypeptides, might be explained by the existence of local membrane heterogeneities and the selection of membrane domains that are involved in the vesicle flow processes, as opposed to other membrane domains that do not participate in secretory membrane translocations (Morré, Keenan & Huang, 1974; Franke & Kartenbeck, 1976).