This study is aimed at a better understanding of the pathogenesis of urinary tract infection (UTI) by examining factors influencing the bacterial ecology of the genital tract. It comprises two sets of experiments in a monkey model. In the first the persistence and transmission between individuals of a P-fimbriated Escherichia coli (strain DS17) in faeces was examined and in the second we studied the influence of amoxicillin on the occurrence of this strain in the vagina. Orally administered E. coli DS17 was shown to spread to cage mates and to persist in the gut for at least 17-18 months. One of four monkeys so colonized developed three separate UTIs with the DS17 strain. The second set of experiments comprised four other monkeys, who either harboured the E. coli DS17 strain in the faeces and/or in small amounts in the vagina, probably through contamination during defaecation. Amoxicillin induced a persistent vaginal E. coli DS17 colonization in nine of ten experiments. The study thus shows that uropathogenic E. coli may persist for long time in the faeces and that, in this situation, amoxicillin may promote an abnormal, vaginal E. coli colonization similar to that characteristic of females prone to recurrent UTI and often preceding manifest urinary infections.