A total of 342 couples planning to undergo in-vitro fertilization (IVF) were examined for the presence of bacteria in semen prior to and during the procedure. Pregnancy rates were analysed retrospectively to ascertain any adverse affects if > 10000 colony forming units (CFU)/ml bacteria were detected in the semen sample. The most common bacteria isolated from semen were Enterococcus spp. (73%). The presence of these bacteria did not affect the pregnancy rate of the patients with positive cultures prior to (32%) or during (37%) the IVF procedure compared with those patients in whom no bacteria were detected (32%). Those patients with semen cultures positive for Escherichia coli prior to the IVF procedure, but which cleared after treatment had a higher pregnancy rate (60%) compared with those patients who were positive for E.coli at the time of the attempt. The group of patients with Staphylococcus aureus in the semen at the time of IVF also demonstrated a low pregnancy rate (17%). Of the patients. 36 (11%) had positive Ureaplasma cultures from the screening test carried out on the semen and 22% became pregnant after successful treatment. None of the three patients with persistently positive cultures became pregnant. In conclusion, the presence of Enterococcus in semen does not affect pregnancy rates following IVF. E.coli, S.aureus and Ureaplasma urealyticum may have a negative effect and should be treated.