The cooperative mechanism of oleic acid/oleate vesicle formation is investigated under a variety of conditions. When a stock solution of sodium oleate is injected in a buffered aqueous solution of pH 8.8, the time progress of the vesicles formation follows a sigmoidal pattern, which is indicative of an autocatalytic process. Autocatalysis is demonstrated by running experiments in the presence of preadded vesicles, which shortens the lag phase and accelerates the process of vesicle formation as measured by turbidity and electron microscopy. The size distribution of the vesicle diameters obtained by injection of the surfactant stock solution in buffer solution covers a wide range, typically between 50 nm and 1.5 mu m. However, in the presence of extruded vesicles having respectively 50 and 100 nm diameter, a quite different situation is observed: the size distribution is then much narrower and strongly biased toward the diameter of the preadded vesicles (i.e., 50 or 100 nm). It is as if the vesicles which are originally present would exercise a kind of template (or matrix) effect upon the new vesicles. This "matrix" effect is studied under a variety of concentration conditions of the preadded vesicles and the added surfactant stock solution. It is also investigated with a heterogeneous system, in which the water-insoluble and overlaid oleic anhydride is hydrolyzed in the presence and in the absence of preadded vesicles, and tendentiously the same matrix effect is observed. It is argued that this is most likely due to a prerequisite binding of the injected surfactant to the already existing vesicles, and it is this binding which determines the final size distribution.