The effects of different heat treatments applied to milk (UHT and pasteurised milk were compared to raw milk) and the addition of 0.1 mM KlO(3) before heating on proteolysis in milk, composition of Cheddar cheese made from the milk, and proteolysis during ripening of the cheese were studied. Little effect of pasteurisation on composition or primary proteolysis of Cheddar cheese during ripening was observed, although free amino acid levels were elevated in cheeses made from pasteurised milks during the later stages of ripening. Addition of KlO(3) to milk before pasteurisation had little effect on composition or proteolysis, suggesting that whey protein incorporation by sulphydryl mechanisms is of minor importance in influencing quality of cheese made from pasteurised milk. UHT treatment, however, altered ripening patterns in cheese considerably, with increased breakdown of alpha(s1)-casein and decreased breakdown of beta-casein by plasmin being observed. This resulted in increased production of water-soluble N, but had little effect on free amino acid levels in cheese. The addition of KlO(3) to milk before UHT treatment had little effect on composition or indices of proteolysis, but increased breakdown of beta-casein, due to a protective action on plasmin, to a level intermediate between that seen in raw and UHT milk cheeses.