In vitro penetration tests of human sperm into cervical mucus were introduced in order to study the interaction between sperm and cervical mucus. In the present study the correlation between sperm velocity and penetration value has been assessed. 197 semen samples were run in the in vitro penetration test using semen of different qualities and cervical mucus of good quality. No differences were found either between sperm velocity in cervical mucus (17.3 +/- 0.9 micrometers/s) and in semen (19.9 +/- 0.8 micrometers/s) or at different distances measured along the flat capillary tube (1, 2 and 3 cm). A highly significant correlation was found between sperm velocity and penetration value rates (p less than 0.0001, r = 0.9). Thus, sperm velocity in semen is a variable of great importance in the assessment of the quality of the ejaculate.