We report a comparative study on protein and enzyme content of human periovulatory cervical mucus and endocervical epithelium. The salient results of our investigation are summarized in the following three points: 1) The method of taking of cervical mucus samples may markedly affect the results of protein and enzyme assays. It is warmly advisable to collect mucus samples from the endocervical canal. 2) Cervical mucus from women diagnosed as having cervical-factor infertility is oligoproteic and lacks some enzyme activities always present in normal cases. 3) Human cervical mucus and endocervical epithelium are very similar as for the enzyme distribution, but show quite different LDH-isoenzyme patterns. In the latter tissue, the H subunits constitute about 77 per cent of the whole LDH, suggesting that this enzyme here is functioning to produce pyruvate rather than lactate.