The relation between sperm antibodies (agglutinins and IF-antibodies) in serum and seminal plasma was studied in 3 selected groups of male partners in infertile couples with respect to specificity, concentrations and immunoglobulin classes. Agglutinins were found in seminal plasma only when they were also present in serum but there was no strict correlation between the titers in seminal plasma and serum although serum titers were always the higher ones. In cases with tail-to-tail agglutinins a difference of more than 2 4-fold steps between the titers was never found; spontaneous agglutination in the ejaculate generally occurred when the seminal plasma titer was 64 or more; in 6 patients with high seminal plasma titers Ig[immunoglobulin]A was detected in the midpiece region of ejaculated spermatozoa. In contrast, head-to-head agglutinins were found in seminal plasma in only 1 of 3 patients with these agglutinins in serum and only in a low titer. Absorption of IgG with protein-A-producing Staphylococcus aureus showed that tail-to-tail agglutinins in serum were IgG antibodies but they could be characterized as non-IgG, probably IgA, in seminal plasma, suggesting a local production of these antibodies. IF-antibodies were rarely found in seminal plasma and seemed to be of little significance in relation to infertility in men.