To study the effect of mycoplasmas and Chlamydia trachomatis infection on semen quality, these microorganisms were cultured from the semen and anterior urethra respectively, in a group of 175 infertile men suspected of a silent genital infection with a poor postcoital test. Chlamydia infection, but not mycoplasmas, was parodoxically more frequent in the apparently normal than oligotetratoasthenozoospermia patients. Mycoplasmas male infection, but not chlamydia, was more frequent in cases with female, mechanical, and/or organic infertility factors. Infection was unrelated to the accessory gland evaluation or sperm variables. However, seminal antisperm antibody activity was significantly increased in cases with any positive culture. By this local antibody increase, chlamydia and mycoplasmas may significantly reduce sperm egg penetration ability.