Previous authors reported high incidences of complement dependent sperm immobilizing antibodies in cervical mucus (CM) and found a correlation with poor postcoital tests. In all those investigations the CM samples were tested after dilution with a salt solution. In the present study we investigated 50 CM samples from infertile couples who had during the routine fertility investigation poor postcoital tests. All CM samples were tested after dilution with an equal volume of normal human serum; only one sample was found positive. On the other hand, if the CM samples were diluted with an equal volume of a buffered salt solution, many positive results were obtained. With some guinea pig serum batches, however, all tests with buffer-diluted CM samples were negative, despite good reaction of the positive control serum. We concluded that the positive results with buffer-diluted CM samples were due to a factor in the animal serum used as complement preparation and that complement-dependent sperm immobilizing antibodies in CM are an unlikely cause of a poor postcoital test.