Effects of high pressure (HP) on average casein micelle size and denaturation of alpha-lactalbumin (alpha-la) and beta-lactoglobulin (beta-lg) in raw skim bovine milk were studied over a range of conditions. Micelle size was not influenced by treatment at pressures <200 MPa, but treatment at 250 MPa increased micelle size by similar to25%, while treatment at greater than or equal to300 MPa irreversibly reduced it to similar to50% of that in untreated milk. The increase in micelle size after treatment at 250 MPa was greater with increasing treatment time and temperature and milk pH. Treatment times,greater than or equal to2 min at 400 MPa resulted in similar levels of micelle disruption, but increasing milk pH to 7(.)0 partially stabilised micelles against HP-induced disruption. Denaturation of alpha-la did not occur less than or equal to400 MPa, whereas beta-lg was denatured at pressures > 100 MPa. Denaturation of alpha-la and beta-lg increased with increasing pressure, treatment time and temperature and milk pH. The majority of denatured beta-lg was apparently associated with casein micelles. These effects of HP on casein micelles and whey proteins in milk may have significant implications for properties of products made from HP-treated milk.