In this study, high pressure (HP)-induced denaturation of alpha-lactalbumin (alpha-la) and beta-lactoglobulin (beta-lg) in dairy systems was examined. in both milk and whey, beta-lg was less baroresistant than alpha-la; both proteins were considerably more resistant to HP-induced denaturation in whey than in milk. HP-induced denaturation of alpha-la and beta-lg increased with increasing proportion of milk in mixtures of milk and whey. Addition of a sulphydryl-oxidising agent, KlO(3), to milk or whey increased HP-induced denaturation of beta-lg, but reduced the denaturation of alpha-la. Denaturation of both alpha-la and beta-lg was prevented by adding a sulphydrylblocking agent, N-ethylmaleimide, to milk or whey prior to HIP treatment, highlighting the crucial role of sulphydryl-clisulphide interchange reactions in HP-induced denaturation of alpha-la and beta-lg. Removal of colloidal calcium phosphate from milk also reduced HP-induced denaturation of alpha-lg and beta-lg significantly. The higher level of HP-induced denaturation of alpha-la and beta-lg in milk than in whey may be the result of the abscence of the casein micelles and colloidal calcium phosphate from whey, which facilitate HP-induced denaturation of a-la and beta-lg in milk.