The inner acrosomal membrane (IAM) develops during the spermatid stage of differentiation as that portion of the Golgi-derived acrosome granule that tightly associates with the condensing sperm nucleus. In some mammalian species, an electron-dense proteinaceous material accumulates between the IAM and the nuclear envelope, collectively comprising the “perforatorium.” Evidence, including its partial purification and its structural resistance to detergents and sonlcation, suggests that the IAM is an unusually resiliant membrane. Dense paracrystalline arrays of intramembranous particles, a lack of lectin-mediated receptor modulation, and its lack of participation in sperm-egg fusion suggest that the IAM lacks the same degree of fluidity as the egg surface plasmalemma. Observations using monoclonal antibodies, however, suggest that some specific antigenic modulations may be possible within the IAM. Its structural rigidity is of obvious mechanical value during sperm penetration through the zone pellucida. An additional role as a scaffold for putative zona lysin material remains controversial. Biochemical evidence suggests that acrosin, for example, is not entirely soluble and that some remains sperm-associated, depending on the conditions of acrosome disruption. Nevertheless, morphological studies do not agree on acrosin's specific localization to the IAM. Currently there is only very limited information concerning the localization of the other acrosomal enzymes to the IAM. Another possible role for the IAM in some species may be in recognizing the zona pellucida. Evidence for this derives from the observation that fucoidin, a fucose heteropolysaccharide, inhibits guinea pig sperm-zona binding, and bound fucoidin can be localized to the IAM and equatorial regions of the living acrosomereacted spermatozoa. Finally, the IAM may have a role in early recognition/adhesion with the colemma.