Amiloride is one of the major molecular probes in basic and applied investigations on the physiology of cation transport in animal cells. In these cells the drug also exerts growth inhibitory activity. Recently, we discovered that amiloride causes growth inhibition also on bacterial cells. In this paper we report that medium pH influences amiloride activity on Streptococcus faecalis. The lowering of external pH causes a drop in the susceptibility of this bacterium to amiloride up to an almost complete resistance. This finding, constitutes a novel aspect of the in vitro experimental pharmacology of this diuretic potentially useful also in clinical pharmacology and in animal cell investigations.