The leucine-responsive regulatory protein (Lrp) is a global regulator that controls the expression of numerous operons in Escherichia coli. Lrp can act as a repressor or as an activator of transcription with its effects being potentiated, repressed or unaffected by the presence of exogenous leucine. The phase variation of type 1 fimbria in E. coli provides a unique system in which to investigate the effects of leucine on Lrp, as it is the only known example in which Lrp is a positive regulator and leucine potentiates this effect. Previous studies determined that Lrp binds with high affinity to two sites within the fim switch (fim sites 1 and 2), and binding to these sites stimulates recombination. Here, it is shown that, even though leucine stimulates the fim switch in vivo, it nevertheless causes a slight decrease in Lrp binding to the fim switch in vitro. These contradictory results are explicable by the finding that Lrp binding to a third region adjacent to fim sites 1 and 2 inhibits recombination. According to this model, leucine stimulates recombination by selectively disrupting Lrp binding to this newly characterized region, while having little or no effect on Lrp binding to fim sites 1 and 2.