The incidence and the clinical relevance of sperm-reactive antibodies in sub-fertile men and in testicular cancer patients were assessed in a pilot study. The sera of 42 men with normogonadotropic oligoasthenoteratozoospermia (OAT syndrome, n = 20) or carcinoma of the testis after inguinal semicastration (n = 22) were analyzed for agglutinating antisperm antibodies using fiuorescein-labeled antiglobulin. In the group with the OAT syndrome, the incidence of sperm-reactive antibodies was only 5 %, which is comparable to that in normal fertile men. Although the incidence of 18% in the testicular cancer patients was markedly higher, only 2 of the patients in question had abnormal spermiograms, which in one case could, moreover, be explained by previous radiation therapy. In summary in this small group of patients, serum monitoring for sperm-reactive antibodies appeared to be of limited clinical relevance in patients with the OAT syndrome and in testicular cancer patients.