Human and bovine cervical mucus penetration tests (n = 57) were performed preceding IVF to test their prognostic value as sperm function tests for IVF. This evaluation also included results from conventional semen analysis and from a computerized sperm analysis system. The bovine cervical mucus penetration test was shown to be at least as valuable as the human cervical mucus penetration test in evaluating sperm function. The migration distance of the vanguard sperm (P less than 0.001) and the sperm density at a fixed migration distance in the mucus column (P less than 0.05) correlated most closely with the IVF results. A clear parallelism with the outcome of the 'swim up' technique was also found. Of the sperm parameters examined, only sperm motility in the ejaculate (P less than 0.05) correlated significantly with the results of IVF. It is concluded that the outcome of a bovine cervical mucus penetration test depends on the same sperm functions as required for IVF. Therefore, this test may be of predictive value in an IVF programme.