Monoclonal antibody (mAb) MN13 labels mouse sperm head postacrosomal perinuclear theca (PT), which is possibly involved in oocyte activation during fertilization. The antigenic site is expressed after mild sonication followed by treatment with dithiothreitol (DTT) or heat (45°C), and is visible as a thick band in the postacrosomal region. The presence of protease inhibitors in the sonication medium suppresses the exposure of MN13 epitope (MN13p), suggesting the involvement of a proteolytic reaction in this process. Spermatozoa do not express MN13p after the induction of acrosome exocytosis by Ca² ionophore, zona binding, or during zona penetration, a strategy that ensures safe delivery of postacrosomal PT proteins to oocytes after fusion. MN13 labeling was not detectable during fertilization by zona-free in vitro fertilization, suggesting that the antigenic site does not react with proteolytic enzymes during sperm-oocyte fusion and the antibody does not recognize the nascent epitope. Microinjection of sperm heads prepared by sonication and DTT treatment led to the activation of metaphase II oocytes. The oocyte activating function of such sperm heads was significantly diminished after labeling with MN13 prior to intracytoplasmic sperm injection (ICSI), but labeling with antiequatorin antibody MN9 activated oocytes with a frequency similar to that of unlabeled sperm heads. The sperm heads in inactive oocytes formed premature chromosome condensations (PCCs), which were invested by independent metaphase-like spindles. These observations indicate that the postacrosomal PT recognized by mAb MN13 is involved in oocyte activation. MN13p is dissociated from sperm heads during the early stages of decondensation after ICSI. In activated oocytes, MN13-labeled fine granules were redistributed in the midzone spindle region, whereas in inactive oocytes they formed a ring around the polar regions of the metaphase II and PCC spindles.