In this study, we quantified secretory leukocyte protease inhibitor (SLPI) and elastase in ejaculates from normal donors and infertile patients with or without leukospermia and investigated the effect of SLPI on sperm motility reduced by elastase. Western blot analysis revealed that SLPI protein was detected in the seminal plasma. The SLPI titre in the seminal plasma with leukospermia was lower than that in the seminal plasma without leukospermia and in the seminal plasma of fertile donors. The elastase concentration in the seminal plasma with leukospermia was significantly higher than that in the seminal plasma without leukospermia. A significant correlation between SLPI and elastase concentrations in the seminal plasma (r = 0.36, P< 0.01) was observed. There was a positive correlation between SLPI titre in the seminal plasma and sperm motility (r = 0.51, P < 0.001). SLPI recovered the sperm motility reduced by elastase in a dose-dependent manner. Our results suggest that SLPI is a potential substance to treat infertile patients with leukospermia.