Over the last 2 years, data on cervical mucus, thawed semen and their interaction in vitro have been collected at the time of insemination in all patients referred to our clinic for artificial insemination by donor (AID). A total of 37 patients have become pregnant, including one who became pregnant twice, and 27 patients have completed a course of 6-12 inseminations or withdrawn without becoming pregnant after a minimum of four cycles of treatment. A comparison of data from these two groups reveals significant differences in cellularity, penetrability and ferning of the cervical mucus and in the motility of spermatozoa in the thawed semen. In a comparison of sperm parameters in the same semen sample prior to freezing and after thawing, those samples which were fertile showed a lower reduction in motility due to freezing and thawing. Studies on the in vitro interaction between mucus and spermatozoa show that, in certain combinations, the mucus appears to induce abnormalities in the midpiece region or the tail, which impair sperm progression. Collection and analysis of data from up to three inseminations per cycle demonstrates that there is considerable variation between patients in the stability of their periovulatory mucus and has also allowed a number of patients with 'problem mucus' to be identified.