Protein carboxyl methylase (PCM) activity was measured in human spermatozoa of 16 normal fertile men and 26 men with various forms of asthenospermia. PCM activities were in the range 14-635 pmol/min per 10(9) sperm. Six men with idiopathic asthenospermia had low PCM activity but the defect of sperm motility was not severe (sperm motility 25-50%). One man with zero sperm motility and sparse mitochondria in the midpiece spiral had high PCM activity. Two men, one with idiopathic asthenospermia and the other with asthenospermia following vasoepididymostomy, also had high PCM activities. PCM activity did not correlate with motility or motility index but there was a correlation between PCM activity and the proportion of immature forms in semen. Thus, the relationship between PCM activity and low motility of sperm is not simple and other cells also contribute to overall seminal PCM activity.