Our study focuses on a foodborne pathogen, Campylobacter, which is responsible for the most frequent bacterial enteritis worldwide. Membrane integrity of Campylobacter jejuni NCTC 11168 cells treated at high pressure (300 MPa, 20 degrees C, 10 min) at pH 7.0 and pH 5.6 was measured by fluorescence spectroscopy of propidium iodide (PI) uptake. The percentage of membrane-damaged cells by high pressure, in which PI is allowed to penetrate, was determined using two calibration methods based on the PI fluorescence signal obtained with cells killed either by a heat treatment (80 degrees C for 15 min) or by a pressure treatment (400 MPa, 20 degrees C, 10 min). Both calibrations were shown to be statistically different (P < 0.05), particularly at acidic pH, suggesting that a difference in the penetration of PI into bacterial cells might depend on the mode of cell inactivation. These results corroborate the fact that the mechanism of microbial inactivation by high pressure is pH-dependent.