Semen characteristics, antisperm antibodies, and cervical mucus penetration studies were analyzed in 754 couples and 95 men undergoing infertility evaluation. The means for the different semen/sperm variables were within ranges published for fertile men. Ages of the men ranged from 22 to 55 years and accounted for a small amount of variation. Sperm counts were lowest in September, December, and January, and highest in April, May, October, and November. Of the sperm characteristics, morphology appeared to be associated with the most other variables. Specimens with more than 50% abnormal sperm forms were overall of significantly poorer quality in terms of sperm counts, motility, forward progression, and ability to penetrate cervical mucus. Antisperm antibodies (agglutinating and immobilizing) were detected in the serum samples of 19.0% of the men, 20.4% of the women, and 32.8% of the couples where one or both partners were positive. Agglutinating antibody titers were significantly correlated between partners. Serum titers of antisperm antibodies were associated with decreased sperm counts, motility, forward progression, and normal forms (immobilizing antibodies). Multiple correlation analysis indicated significant independent effects of sperm concentration, motility, forward progression, and antibodies on sperm-cervical mucus penetration scores of the men. In women, cervical mucus penetration was adversely affected by the presence in the serum of sperm agglutinating antibodies and of immobilizing activity in the cervical mucus.