The activities of angiotensin-converting enzyme (ACE) and leucinaminopeptidase (LAP) are positively correlated with corresponding concentrations of sperm cells in semen of boars kept under normal conditions. The spermatozoa bound ACE activity, in general, does not reflect differences in the quality of semen (bull and boars). On the other hand, the ACE activity directly bound on the sperm cells is significantly elevated, if 'exogenic noxes' (by feeding or keeping) influence the fertility of boars in a drastic manner. These results are discussed with regard to the differential diagnostic importance for estimating the semen quality and to the causal relations between increased enzyme binding and injury of sperm cells.