Seven couples who were in a program of artificial donor insemination (AID) which had not resulted in a pregnancy were advised to abstain from sexual intercourse for 2 days prior to AID. Six of the wives became pregnant following this advice. Seminal plasmas from the seven husbands showed evidence of spermatozoal antibodies. They agglutinated or immobilized healthy spermatozoa and contained immunoglobulin A and complements C-3 and C-4. Immunoglubulin G levels in these seminal plasmas were slightly elevated. Blood sera from the same husbands also agglutinated normal spermatozoa or decreased their motility. A comparison of 33 women who were not advised concerning sexual intercourse prior to AID and 17 women who were advised revealed that 46% of the former group became pregnant, whereas in the advised group 88% achieved pregnancy. The conclusion is that abnormal semen may contain spermatozoal antibodies which adversely affect normal spermatozoa. In order to avoid the presence of spermatozoal antibodies on the wife's cervix, couples are advised to abstain from sexual intercourse, or to use a condom, for 2 days prior to AID.