A microbiological examination has been carried out in 116 patients with unexplained infertility and with asymptomatic bacteriospermia. Organisms more frequently isolated were Staphylococcus epidermidis (81.9%), non hemolytic streptococci (23.3%), diphtheroids (25%) and alpha-hemolytic streptococci (18.1%). None of the 103 patients were positive for C. trachomatis. Mycoplasmas were isolated in 56 (48.3%) of the 116 examined samples, U. urealyticum was present in 49 (42.2%), M. hominis in 3 (2.6%) and both species in 4 (3.5%) samples of examined fluids. Forty-one of the 56 mycoplasmas-positive patients have been treated with doxycycline therapy. The treatment improved motility and caused decrease of coiled tails in 12 cases. Conception occurred in 5 (26.3%) of the 19 treated patients whose cultures were negative for mycoplasmas. These 5 pregnancies occurred in the patients that had high titers (greater than 10(5) c.c.u./ml) of ureaplasmas and in which the antibiotic therapy was successful. No pregnancy was seen in the other 22 patients where treatment failed to eradicate mycoplasmas.