It was suggested that although not related to the standard semen parameters the level of the acrosin enzyme system is related to the fertility potential in men. Recently a simple clinical assay for total acrosin level was recommended for routine semen analysis. The improved clinical assay was analysed on the freshly liquified semen of 198 Asthenoteratozoospermic men and compared with the routine semen parameters including biochemical data and the ultrastructure of the acrosome. Only the sum of the per cent of live, motile, and normal-shaped spermatozoa had positive significant and reasonably high correlation with the acrosin level (r = 0.382, P < 0.0001). Each characteristic exhibits significant but low (< 0.35) correlation. Similarly negative significant and reasonably high correlation was obtained between the acrosin level and the sum of the principle acrosomal malformations observed by TEM (r = 0.396, P < 0.0001) while lower negative correlation was found only with agenesis or loss of the acrosome. Acrosin levels below 8.1 microIU 10(-6) cells were obtained in 4 specimens with above 80% round-form associated with more than 95% of agenesis of the acrosome. The possible significance of the low correlation obtained between the acrosin levels and seminal plasma zinc levels, malformations in the acrosomal equatorial region, and the presence of white blood cells is also discussed. We concluded that the acrosin activity reflects an aspect of male fertility which is not diagnosed by the routine semen analysis or by the ultrastructure of the acrosome, and is therefore a useful diagnostic sperm parameter.