Fifty-two sterile couples, characterized by persistent and unexplained incompatibility between spermatozoa and cervical mucus, without any other evident factor to explain their sterility, were investigated for the possible role of blood group incompatibility as a cause of their sterility. After a study of the distribution of blood groups in sterile and fertile couples, the distribution of blood group substances in pairs of husbands and wives, and the effect of blood group antibodies on sperm migration, it seems highly unlikely that blood group incompatibility contributes significantly to cervical hostility and the resulting sterility.