Rhesus monkeys develop agglutinating and complement-dependent antisperm antibodies after vasectomy. In order to study whether these antibodies affect fertility after vasovasostomy, 15 animals were given vasectomies and 6 months later vasovasostomies. Subsequently, each was mated with females of proven fertility. Five controls were given sham operations and similarly treated. During this period, each aimal was bled for serum to monitor the humoral immune response, ejaculated for semen analyses, and palpated for granuloma or fistula development. All control animals had a transient decrease in sperm density after sham vasectomy and vasovasostomy operations. The surgical procedures of vasectomy and subsequent vasovasostomy resulted in more animals having sperm of poor motility and quality. All of vasovasostomies were surgically successful in that sperm were again present in the ejaculate of each animal. The amount of sperm in the ejaculate could not be correlated with the ease of surgical procedure, presence or absence of macrophages in the ejaculum, motility, or forward progression. Only animals that had been vasectomized developed circulating antisperm antibodies. Sustained, elevated levels of antisperm antibodies most commonly occurred in monkeys that had high initial total sperm counts. Six of the experimental animals retained high levels of sperm-immobilizing antibodies after vasovasostomy. Of these, two were found to be infertile and two were classed as subfertile. Of the nine experimental animals without sustained antisperm antibody production, only one was classed as subfertile. This suggests that antisperm antibodies may in some cases impair the restoration of fertility after vasovasostomy.