The concept of infertility as a result of asymptomatic microbial colonization of the female reproductive tract has been neglected to date. However, increasing incidence of infertility and advanced research has drawn attention towards this idea. Many of these micro-organisms have been reported to bring about adverse changes in sperm parameters in vitro, but their in vivo potential to cause infertility is still a controversy. The present study was carried out to observe what effect the intravaginal inoculation of sperm-agglutinating Serratia marcescens and sperm-immobilizing Candida albicans had in the reproductive tract and consequently in fertility outcome. When these strains were intravaginally inoculated into female BALB/c mice at 104, 106 and 108 c.f.u. in 20 µl PBS for 10 consecutive days, with mating of mice on day 12, the results showed 100 % decrease in fertility in all groups as compared with control mice receiving PBS alone. Furthermore, no clinical or histopathological changes were observed in the reproductive organs (i.e. ovary, uterus and vagina), suggesting that colonization of the genital tract with sperm-impairing micro-organisms could be a feasible reason for female infertility.