30 asthenozoospermic subfertile men were treated parenterally for 3 months with kallikrein. 20 men showed a significant increase in sperm motility, 66% (responders), while the other 10 men, 34% (non-responders), did not respond to this treatment. A striking decrease in sperm concentration (by 32%) was found in most patients. No latent period was found following kallikrein treatment. Thus, semen quality did not differ if examined immediately following the cessation of treatment or a few weeks afterwards. A poor pregnancy rate was recorded, i.e. 20% among the responders and 10% among the non-responders. In conclusion, kallikrein is a useful drug in the treatment of asthenozoospermia only when the sperm concentration is not decreased below the optimal value.