Spores of Clostridium sporogenes were found to be resistant to ultra high pressure, with treatments of 600 MPa for 30 min at 20 degrees C causing no significant inactivation. Combination treatments including heat and pressure applied simultaneously (e.g. 400 MPa at 60 degrees C for 30 min) or sequentially (e.g. 80 degrees C for 10 min followed by 400 MPa for 30 min) proved more effective at inactivating spores. Pressure cycling (e.g. 60 MPa followed by 400 MPa at 60 degrees C) also reduced spore numbers. Overall, these pressure treatments resulted in less than a 3 log reduction, and it was concluded that the spores could not be inactivated by pressure alone. This could indicate that for the effective inactivation of bacterial spores, high pressure technology may have to be used in combination with other preservation methods.