The influence of four different copper-containing intrauterine devices (IUD) (Nova-T, Multiload, Fincoid 250 and copper-T 250) on the penetration of human sperm into bovine cervical mucus were assessed. Pooled samples of predetermined sperm concentration were suspended in Earl's medium in which a copper IUD was previously incubated for periods between one hour to fourteen days. The mean copper concentration was determined for each of the medium containing IUD and was found to be between 284 +/- 93 micrograms/100 ml to 392 +/- 138 micrograms/100ml. While there was no adverse effect on sperm motility by the copper-containing medium, there was a significant reduction in the number of sperm penetrated into the bovine cervical mucus as compared to the penetration of sperm suspended in pure Earl's medium. It therefore seems that the influence of copper on sperm penetration might be by an effect on the environment or spermatozoal migration rather than by direct effect of copper on sperm motility.