In order to evaluate the significance of sperm antibodies in relation to infertility, studies have been made on both infertile and fertile populations. The infertile population consisted of about 150 couples with long-standing infertility, predominantly of the unexplained type. The fertile population consisted of 78 antepartum women and 35 postpartum women. In all these individuals, sperm antibody was sought by two methods of serum testing, namely, the Kibrick (K-B-M) and the F-D methods of sperm agglutination. In the infertile group, the Kibrich test was positive for 23.1 per cent of the women and 9.6 per cent of the men, whereas the F-D test was positive in 16.4 per cent of the women and 5.0 per cent of the men. In contrast to these values, it was found in the fertile group that the Kibrick test was positive in 2.7 per cent of these women and the F-D test was also positive in 2.7 per cent of them. A number of technical details and precautions have also been emphasized. These principles are intended to minimize the occurence of nonspecific and false-positive results.