The acrosome reaction (AR) is a prerequisite for successful fertilisation. However, little IS known about processes occurring in spermatozoa before and during the AR. The AR involves fusion of the outer acrosomal and plasma membranes, resulting in both the release of acrosomal contents that might facilitate per ration of the zona pellucida (ZP) and the exposure of the inner acrosomal membrane lAMl which contains membrane components required for gamete fusion. The membrane complement regulatory (CReg) protein CD46 is expressed by spermatozoa solely on the 1AM and not the plasma membrane, and can be used as a marker for the AR. Vhe initial aim of this study was to characterise the protein expression of CD5 5 and CD59 on human spermatozoa using CD46 as a tool for detecting the AR.