The consistency index of cervical mucus has been studied throughout the menstrual cycle in a group of 125 women. An attempt has been made to correlate consistency with other cyclical changes in mucus such as recoil, palm leaf reaction and tests for glucose content. The limits of consistency within which spermatozoa will move freely in mucus have been established from post-coital samples and invasion tests in vitro. The characteristics of twenty-four mucus samples obtained on the day when conception is known to have taken place are discussed.

Summary. The