In preliminary experiments, pH measurements were made with pH paper and with a pH electrode on cervical mucus in situ and after aspiration. Paper and electrode gave comparable results with aspirated mucus. Measurements made on mucus in situ and then after aspiration were concordant only when pH paper, but not the pH electrode, was used to measure pH in situ. All spermatozoa were immotile in the post-coital test when the pH of the mucus was below 6. A pH below 6 was found in 38% (23 out of 61) of post-coital tests in which there were fewer than 1% of motile spermatozoa (19% of all tests). The male partners of women who had a mucus pH greater than 6 but had less than 1% of spermatozoa motile in the post-coital test, had slightly poorer quality semen than the partners of women with mucus pH less than 6, or in which there were motile spermatozoa in the mucus.