Acrosin is a trypsin-like serine protease present in its zymogen form in the acrosome of spermatozoa. The activated enzyme is thought to digest a pathway for the sperm through the zona pellucida of the ovum during the process of fertilisation. We have shown that in a purified system boar acrosin was inhibited by human protein C inhibitor with an apparent second order rate constant (kapp) of 3.7×104M-1s-1 1. Protein C inhibitor is present in high concentrations in seminal plasma and endogenous protein C inhibitor was found in the immediate vicinity of disrupted acrosomal membranes of washed human spermatozoa. This serpin could therefore function as a scavenger of prematurely activated acrosin in the male reproductive tract. Since little is known about the interaction of acrosin with other serpin type inhibitors, we analysed in this study the interaction of boar acrosin with other purified human serpins. Antithrombin III, plasminogen activator inhibitor-1, plasminogen activator inhibitor-2, and α1-antitrypsin inhibited acrosin activity. The following apparent kapps were calculated: Antithrombin III: 19.5×104 M-1s-1, plasminogen activator inhibitor-1: 21.5×104M-1s-1, plasminogen activator inhibitor-2: 3.3×104M-1s1, α1-antitrypsin: 0.09×104M-1s1. α2-antiplasmin and heparin cofactor II did not inhibit acrosin. SDS-stable acrosin/serpin complexes were only seen with antithrombin III; all other acrosin inhibitors were cleaved by the enzyme. As determined by enzyme-linked immunosorbent assays, the concentrations of protein C inhibitor, plasminogen activator inhibitor-1, and plasminogen activator inhibitor-2 in individual seminal plasma samples from healthy donors were 5.33±0.47 μM, 88±24 pM and 163±17 pM (means±SD), respectively. The concentrations of antithrombin III in seminal plasma were ≤50 nM (semiquantitative immunoblotting). Therefore considering both, the kapp calculated for the inhibition of boar acrosin in a purified system and the concentration of each serpin in seminal plasma, protein C inhibitor seems to be the best candidate to function as a physiological acrosin inhibitor in the male reproductive tract. © 1994.