Mutants defective in phoB, the positive gene activator of the Escherichia coli pho regulon, exhibit aberrant behaviour on MacConkey indicator plates. They appear pale in the presence of a fermentable carbon source such as trehalose, maltose or glucose. The addition of at least 5 mM phosphate corrects this defect. Colonies of phoB+ strains turn red on MacConkey indicator plates and derepress the pho regulon when the cells are able to ferment the carbon source. In contrast, the inability to ferment the carbon source maintains the pho regulon in the repressed state.