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Growth of enteropathogenic Escherichia coli E2348/69 was inhibited by bicarbonate in a dose-dependent manner, showing approximately 5% growth reduction at 5 mmol l−1 while kanamycin at 3·12 μg ml−1 inhibited growth by 15%, yet when kanamycin and bicarbonate were combined at these concentrations, inhibition increased to 80%. Unexpectedly, at bicarbonate concentrations >20 mmol l−1 enhancement of the antibiotic activity virtually disappeared, i.e. there was a paradoxical Eagle-like effect. How bicarbonate acts is unclear, but neutral or alkaline pH also enhanced the activity of kanamycin. However, several differences indicated a separate effect of bicarbonate. First, bicarbonate inhibited growth more than the corresponding increments in pH. Second, at low concentration, the antibiotic enhancing effect of bicarbonate was stronger than the effect of pH alone. Third, 5 mmol l−1 bicarbonate significantly enhanced the activity of kanamycin while the corresponding pH had no effect. Fourth, the Eagle-like effect was exclusive of bicarbonate because changes in pH did not induce an analogous behaviour. Notwithstanding the mechanism, the enhancing effect of bicarbonate was indubitable. Consequently, it seems worthwhile to explore further its potential to improve the efficacy of aminoglycosides and maybe even other antibiotics.