To determine the distribution of results of an in-vitro cervical mucus penetration system employing both human and bovine mucus (Tru-Trax) in a general infertility population, 133 couples prospectively underwent in-vitro and post-coital testing of cervical mucus. The distribution of Tru-Trax results in couples with normal semen analyses and Insler scores showed significantly greater penetration in bovine (22.3 +/- 6.0 mm) than in human mucus (20.3 +/- 5.4 mm) (P < 0.001). However, the lower limit of the 95% confidence interval of the normal population with either type of mucus was approximately 10 mm. This limit was significantly lower than that described by the kit manufacturer. The predictive value for post-coital tests using human mucus in the Tru-Trax system was good (> 90%) in all groups. The overall penetration into either human or bovine mucus was significantly correlated with the percentage of motile spermatozoa in the semen sample (P < 0.001). In conclusion, in-vitro cervical mucus penetration tests with the Tru-Trax system are related to in-vivo post-coital tests, although the lower limit of the range of anticipated results in the normal population was significantly lower than that described by the manufacturer. The ultimate value of this type of testing will await clinical trials which evaluate clinical end-points such as pregnancy rates.