To further our Knowledge on the mechanisms and molecules involved in mouse sperm–oocyte plasma membrane interaciton, exteraction, experiments were carried out to determine the stage during oogenesis at which an oocte acquires the capacity ot fuse with acrosome-reacted sperm. Zona-ferr oocytes 10 μm in diametes do not fuse with sperm. Oolemma fusibility is first acquired when the oocyte reaches about 20μm in diameter. Fusibility is maniatained even after fertilisation has accurred and is lost completely by the 4–cell stage.