Semen analysis was performed on 226 ejaculates by an integrated microcomputerized system employing the multiple-exposure photography (MEP) method. Mucus penetration tests were performed in vitro using commercial preparations of bovine cervical mucus. A highly significant (P less than 0.001) correlation between mucus penetration distance and sperm count (r = 0.582), motility (r = 0.357), velocity (r = 0.569), motile density (r = 0.582), motility index (r = 0.467), and morphology (r = 0.383) was observed. Increased percentages of immature germ cells (r = -0.318) and bent-tailed sperm (r = -0.221) were the most strongly correlated with mucus penetration. Approximately 10% to 15% of patients with otherwise normal semen parameters displayed poor penetration of mucus. Conversely, 5% to 40% of patients with abnormal semen parameters displayed excellent penetration of the mucus. Motile density and velocity demonstrated the strongest relationship with the outcome of the mucus penetration test. These results suggest that a significant subpopulation of patients can be identified as having inadequate (or adequate) penetration of mucus with otherwise normal (or abnormal) motility characteristics.