Almost one-quarter of 100 asymptomatic men under fertility investigation had significant titres of IgA antibodies (greater than 1:8) specific for Chlamydia trachomatis in seminal plasma. No clear association was evident between the presence of these antibodies and sperm quality. In a second study, the female partners of men with consistently high serum or seminal plasma levels of chlamydia-specific IgA also exhibited a positive IgA reaction in serum without any clinical indications of the presence of an infection. In one group of men and women, antibiotic treatment for 4 weeks resulted in the disappearance of the IgA from the serum after a variable period of less than or equal to 12 weeks. The study indicates that chronic asymptomatic infections with Chlamydia trachomatis may be responsible for a large number of cases of infertility. It also implies that all men and women under investigation for infertility should be routinely screened with chlamydial serology regardless of previous history and clinical findings.