An intact organelle, the prostasome, is secreted by the acinar epithelial cell of the human prostate gland. The ultrastructural location of the prostasome is within membrane-bound storage vesicles in the epithelial cells. Prostasomes are delivered into the glandular lumen by an exocytotic event, which is preceded by fusion of adjacent membranes belonging to the storage vesicle and the epithelial cell. Alternatively, the storage vesicle can be translocated in toto from the cell interior into the acinar lumen through the plasma membrane. This latter event has been designated diacytosis. Both phenomena seem to occur with approximately equal frequency in the human prostate gland. An ATPase system that is Mg2+ and Ca2+-dependent is firmly linked to the membranes encasing the prostasomes. The ATPase system may be the molecular basis for vectorial transport of calcium into these organelles. Also a protein kinase activity is located in the membranes. An increase in membrane thickness was observed on phosphorylation. The physiologic function of the prostasomes is not known. They may be important for promoting forward motility of spermatozoa.