Although many therapies have been advocated in the treatment of idiopathic male infertility, the results of treatment are poor. This probably seems to be due to a lack of one or more proteins constituting the key structures of the spermatozoa. We evaluated the functional structures of the spermatozoa in 11 infertile patients whose semen showed severe oligoasthenozoospermia with immunochemical method and found a case with spermatozoa lacking acrosin. Evaluation of the spermatozoal defect with immunochemical method is desirable in patients with severe oligoasthenozoospermia, especially in cases unresponsive to medical therapy.