Thirty-two couples participating in an in vitro fertilization (IVF) programme were evaluated as regards the prognostic value on fertilization of spermatozoal performance through flat capillary tubes filled with standardized midcycle bovine cervical mucus (Penetrak, Serono Diagnostics, Surrey, UK). A statistically significant correlation (P < 0.033) was observed between the distance travelled by the neat spermatozoa in the mucus and the % penetration of oocytes at IVF. There were also significant correlations between motility and progression (P < 0.004) and a borderline correlation between progression and the Penetrak results (P < 0.098). There was no significant difference between the Penetrak distances travelled between the 9 who conceived (33.4 mm) and the 23 who did not (29.9 mm). While the test does add to the knowledge of fertilization potential, the results extrapolated to a larger series would give false positive rates of 25% and false negative rates of 11%. The absence of a clear end point renders the Penetrak mucus penetration test insufficiently accurate to be used as a main measure of the male factor when advising for or against IVF therapy.