Regular condensation of sperm chromatin during spermiogenesis, which allows decondensation after penetration of the oocyte, is a prerequisite for successful fertilization. During spermiogenesis particular nucleoproteins (histones) are normally replaced by different proteins (protamines). In the case of disturbed chromatin condensation the histones persist, which can be demonstrated by aniline blue staining. This method should be performed prior to IVF because of the significant correlation between fertilization rate of spermatozoa in IVF and normal chromatin condensation.