To investigate the effect of antisperm autoantibodies on the process of in vitro fertilization, sperm immobilization test and sperm agglutination test with serum, and direct immunobead test with semen were conducted on the male partners of 101 infertile couples. Fourteen patients were positive in at least one of these three antisperm antibody tests. Six of them were associated with a low fertilization rate (= LFR group, less than 50% of mature oocytes were fertilized), while the other 8 patients had a normal fertilization rate (= NFR group). The spermatozoa in the LFR group were bound to immunobeads of both IgG and IgA classes. In contrast, the spermatozoa in the NFR group were not bound to IgA. The results of the tests with serum were not related to the outcome of fertilization. By rapid dilution and washing of the semen containing antisperm autoantibodies, the percentage of spermatozoa bound to IgG decreased significantly. However, the rate of fertilization was not improved. Six pregnancies were achieved after 13 embryo transfers in the 14 patients. We conclude that the fertilization rate is reduced when spermatozoa are bound to both IgG and IgA antisperm autoantibodies, or to IgA alone. Antisperm autoantibodies do not seem to hamper embryonal development and implantation.