A nonlinear regression analysis was used in order to fit a logistic model to 200 runs of human ejaculated spermatozoa penetrating cervical mucus of good quality. The data revealed excellent correlation between the number of sperm penetrating (sigma penetration, SP) and the motility (r = 0.776) and vitality (r = 0.534) of the spermatozoa. The percentage of abnormal spermatozoa found in the ejaculate was negatively correlated (r = -0.649), while sperm concentration showed a poor correlation with SP (r = 0.327). Furthermore, concentration was shown to have no effect in samples containing over 5 million sperm/ml. The conclusion of this study is that the two major parameters of sperm quality determining the capacity of spermatozoa to penetrate cervical mucus are mortility and the presence of normal forms of spermatozoa.