The in vitro penetration ability of human sperm into bovine cervical mucus after washing and suspension of the sperm in Earl's medium was compared with that of untreated sperm. The extent of penetration was found to be significantly lower in sperm washed and suspended in Earl's medium than in the unwashed sperm (p less than 0.0001); it increased to some extent on resuspension of the washed sperm in the seminal plasma but still remained significantly lower than that of intact sperm (p less than 0.009). The introduction of caffeine into the Earl's medium resulted in an increased penetration of the washed sperm into bovine cervical mucus, but this was still significantly lower than the penetration of the unwashed sample (p less than 0.02). The data point to the effectiveness of the seminal plasma as a medium facilitating sperm penetration through bovine cervical mucus.