Species of Pseudomonas are among the most important spoilage bacteria in raw milk, due to the production of very heat-stable proteinases, which are able to hydrolyse caseins, even at low temperatures. In this study, the effect of high-pressure (HP) treatment on raw milk microflora and, in particular, Pseudomonas fluorescens DSM 4358 and its extracellular proteinase, was examined. An overall reduction of raw milk microflora was obtained by pressure, with yeasts and moulds being most sensitive and enterococci being resistant up to 500 MPa. Complete inactivation of Ps. fluorescens inoculated into skim milk was obtained after treatment at 300 MPa. A cell-free supernatant of Ps. fluorescens DSM 4358 was prepared and then treated at pressures up to 800 MPa for 10 min at 20 degrees C. The effect of the treated and untreated proteinase in the cell-free supernatant on sodium caseinate during incubation at 37 degrees C for up to 48 h and hydrolysis of caseins was analysed by electrophoresis. While the bacterium was very sensitive to HP, the enzyme was very resistant, continuing to hydrolyse the caseins even after HP treatment at 800 MPa. These results show that the proteinase from Ps. fluorescens DSM 4358 is extremely resistant to HP, which may be of significance for dairy products made from HP-treated milk.