The acrosome reaction that occurs after sperm capacitation, is an exocytotic event induced by a Ca++ influx. It plays an essential role during fertilization, by making spermatozoa able of penetrating the zona and capable of fusing with the egg plasma membrane. Zona pellucida is the natural inducer of the acrosome reaction. Binding of the sperm receptor to ZP3, a zona glycoprotein acting as ligand, triggers the molecular events leading to acrosomal exocytosis. G-proteins may be involved in the signal-transduction pathway during the acrosome reaction. Other known inducers of the human acrosome reaction include follicular fluid, progesterone and the calcium ionophore A23187. Progesterone acts through a receptor on the sperm plasma membrane, while the ionophore promotes non-physiological sperm Ca++ uptake. Several cytochemical procedures have been proposed for evaluating the acrosome reaction: the acrosomal status can be observed after staining or after labeling with lectins or antibodies. These methods both attempt to evaluate sperm viability and to distinguish degenerative acrosomal loss.