Anti-sperm antibodies (ASA) are produced when the blood-testis barrier is disrupted by obstruction, infection or traumatism. Immunoglobulines of the IgG class are usually present in both serum and semen, and the IgG antibodies attach to the spermatozoa. ASA of the IgA class are secreted by the accessory sex glands and are found in semen only. ASA of the IgG class may be detected in seminal plasma, but there is usually no free IgA in seminal plasma. Anti-sperm antibodies of the IgA class almost never occur without IgG antibodies. In case more than 40% of motile spermatozoa present ASA of the IgG class, complementary testing for IgA antibodies is advisable. The immunological factor reduces the probability of natural conception, and this is particularly true when ASA of the IgA class are also present.