A study has been made of the effect of hexachlorophene (2, 2′-methylenebis [3,4,6-trichlorophenol]) and certain related compounds on the dehydrogenases and cytochrome oxidase systems of Bacillus subtilis and Escherichia coli. Hexachlorophene (G-11), the dichloro analog (G-4), and the compound G-11S, which contains a sulfur bridge rather than a CH2-bridge, all at very low concentrations effectively inhibit the glucose, lactic, and succinic dehydrogenases as well as the cytochrome oxidase systems of both organisms. The absence of chlorine from the molecule results in a profound decrease in inhibitory power against the dehydrogenases with a lesser decrease in activity against the cytochrome oxidase. The inhibitory action of these compounds does not appear to be exerted through their action on the sulfhydryl groups of the enzymes nor could evidence be obtained that they compete with the cytochrome system as hydrogen acceptors. Both organisms bind relatively large amounts of inhibitor.