ABSTRACT: By immunoaffinity chromatography using anti-IgG, anti-IgM, and anti-IgA coupled to CNBr-activated sepharose 4B, the immunoglobulin class of sperm-agglutinating antibodies was investigated in cervical mucus from four infertile women. In all patients, it was found that the sperm antibodies in cervical mucus belonged to the IgA class, whereas in serum, which was studied in two of the patients, IgG sperm antibodies were demonstrated. Absorption of the four cervical mucus samples with anti-secretory component sepharose 4B revealed that the IgA antibodies in at least two of the samples were SC-IgA antibodies. Investigation of a third sample by sucrose gradient ultracentrifugation revealed that the IgA sperm antibodies were characterized by a sedimentation coefficient between 9S and 13S, strongly suggesting the presence of SC-IgA antibodies. Accordingly, the sperm-agglutinating antibodies were SC-IgA antibodies in at least three of the four samples studied.