The intracellular pH changes induced in the Gram-positive skin bacterium Propionibacterium acnes by blue light and UVA radiation were studied. Two methods (31P nuclear magnetic resonance (NRM) spectroscopy and fluorescence spectroscopy using a pH-sensitive fluoroprobe (2',7'-bis-(2-carboxyethyl)-5-(and -6-)-carboxyfluorescein)) were used to determine the intracellular pH. The pH changes induced by irradiation were found to be a function of cell survival. These changes as a function of cell survival followed the same pattern for blue light and UVA radiation. A reduction of the pH gradient across the cell membrane (inside alkaline) was found for lethal doses (less than 15% survival). This reduction corresponded to a decrease in intracellular pH and may indicate a proton influx. An increase in the pH gradient, which corresponded to an increase in the intracellular pH, was observed for sublethal doses. This increase appears to be reversible. Thus two separate mechanisms, which appear to be the same in UVA and blue light regions, may be responsible for the irradiation-induced pH alterations.