Protein C inhibitor (PCI) is a non-specific serpintype inhibitor (Suzuki et al., 1989) present in plasma and many other body fluids and secretions (Geiger, 1988; Laurell et al., 1992). PCI is synthesized throughout the male reproductive tract and high concentrations are found in seminal plasma (Laurell et al., 1992). In seminal plasma PCI has been shown to interact with prostate specific antigen (Christensson and Lilja, 1994). Additionally PCI inhibits the sperm protease acrosin and is immunocytochemically localized to disrupted acrosomal membranes of washed sperms, while intact sperms are negative for PCI-antigen (Zheng et al., 1994). Therefore, by inhibiting prematurely released and activated acrosin in the male reproductive tract PCI might protect spermatozoa, surrounding tissues and seminal plasma proteins from proteolytic damage. Under physiological conditions, however, acrosin is activated and released only in the female reproductive tract in the immediate vicinity of the ovum and is thought to be involved in the fertilization process by digesting a pathway for the sperm through the zona pellucida of the ovum. Inhibition of acrosin by PCI might therefore have an influence on the sperm/egg interaction. It was our aim to study PCI synthesis in the female reproductive tract and to analyze the effect of PCI on fertilization. For this purpose we used a mouse in vitro fertilization model.