ABSTRACT: A fertile man had sperm-agglutinating activity in his serum (titers 1:16-1:128) and in his seminal plasma (titers 1:128-1:2048). The antibodies in the seminal plasma could be absorbed with anti-IgA antiserum but not with anti-IgG antiserum. A fresh ejaculate showed strong auto-agglutination of the spermatozoa. With mixed antiglobulin reaction tests (MART) and/or immunobead tests (IBT), IgA and IgG were detected on almost all motile spermatozoa; the erythrocytes, in the MART, and the latex spheres, in the IBT, adhered mainly to the tip of the tail. After mixing the fresh semen with cervical mucus, only 40% of the spermatozoa were locally shaking. The spermatozoa showed excellent penetration of cervical mucus in vitro. This case shows that IgA coating of the tails of the spermatozoa does not necessarily lead to adherence of these spermatozoa to the micelles of the cervical mucus and that the sperm cervical mucus contact test has a better predictive value than the sperm agglutination titer in the seminal plasma.