The sera of 110 couples with unexplained infertility were examined for anti-sperm immobilizing antibodies with the aid of multiple-exposure photography. The sera of 10 females and 3 males had immobilizing activity. Eleven pregnancies occurred in 10 female patients in the group without immobilizing antibodies. None of the 13 females in the couples with immobilizing activity conceived. A negative correlation was found between the occurrence of serum immobilizing antibodies and incidence of pregnancy. No such correlation could be found between the occurrence of autoagglutination in the male's ejaculate and the female partner conceiving. In light of detection of anti-sperm antibodies in a patient with habitual abortions, a possible immune mechanism is suggested.