We questioned the policy of routine microbiological culture of semen prior to in-vitro fertilization (IVF) with a view to prescribing antibiotics to reduce the risk of introducing seminal infection into the embryo culture system. An initial retrospective study examined semen microbiology reports of 449 couples undergoing IVF or gamete intra-Fallopian transfer (GIFT). In semen samples taking ��� 1 days to reach the microbiology laboratory compared with same-day delivery there was increased frequency of significant culture of enterococci (27 versus 15%, P < 0.01). In samples taking ��� 2 days there was increased frequency of significant culture of Gram-negative bacilli (31 versus 12%, P < 0.01) and of overall culture of other potentially pathogenic organisms (26 versus 14%, P < 0.01). We questioned diagnostic accuracy and relevance. Therefore, in a prospective study, semen and high vaginal swabs obtained on the day of oocyte collection were cultured from 100 couples having IVF or GIFT, of whom 52 male partners had been treated with antibiotics following positive pre-IVF semen culture. The presence of bacteria in semen samples used only for IVF (n = 90) did not reduce fertilization rates nor lead to infection of the embryo culture system. However, there was an increased incidence of significant culture of vaginal Gram-negative bacilli in patients with treated partners compared with untreated partners [15/52 (29%) versus 5/48 (10%), P < 0.05]. Thus antibiotic therapy in the male partner may increase the likelihood of inoculation of antibiotic-resistant pathogenic bacteria from the vagina into the embryo culture system during vaginal oocyte collection. In asymptomatic patients, microbiological screening of semen samples prior to IVF treatment and subsequent treatment with antibiotic therapy in those with positive cultures appears to be unnecessary and may be detrimental to IVF outcome.