Fifty-five men have undergone microsurgical vasovasostomy with at least a 1-year follow-up. Serum samples from these men were obtained in the preoperative and postoperative periods, and titers of agglutinating and immobilizing antibodies were measured and evaluated in relationship to subsequent fertility. Of the 35 patients in whom adequate serum and semen samples were obtained over a period of 1 year, 17 men proved their fertility and 18 patients in the allotted time span had not yet fathered any children. No statistically significant difference in the level of antibody titers could be found between the fertile group and the infertile group. Seminal antibody titers were determined in 25 men. Five of these men have thus for impregnated their wives. Only a small number of men were found to have positive titers for agglutinating and/or immobilizing antibodies in their semen. The presence or absence of spermagglutinating and/or sperm-immobilizing antibodies in the sera of men who have undergone vasectomy reversal does not seem to interfere with subsequent fertility.