The effects of acetic, lactic and citric acids and pH on the growth and intracellular pH (pHin) of Listeria monocytogenes Scott A were documented and compared for total acid concentrations ranging from 50 mmol ml-1 to 250 mmol ml-1 for acetic and lactic acids and from 25 mmol ml-1 to 100 mmol ml-1 for citric acid. Initial pH values ranged from 4.7 to 6.0. Although the growth rate of L. monocytogenes Scott A was slower when incubated at 25 degrees C than at 37 degrees C, the relative acid and pH inhibition was identical at both temperatures. As the initial pH values decreased and/or the total acid concentrations increased, the growth rates of L. monocytogenes Scott A decreased. Compared at equal initial pH values and on an equimolar total acid basis, the relative inhibition effect was generally acetic > lactic > citric. When based on initial undissociated acid concentrations, the inhibition effect was citric > lactic > acetic. The effect of differing acid and pH environments on pHin was determined. At equimolar total acid concentrations, the pHin of the cell was changed the least by citric acid and the most by acetic acid. Growth rates were influenced by the pHin and the acid used to adjust the system.