In this study the effect of heat treatment of milk on the fundamental susceptibility of casein in milk, and beta-casein in particular, to proteolysis by plasmin, was examined by studying the patterns of proteolysis resulting from digestion of milk casein by exogenous plasmin added, after heating, to final concentrations sufficiently high as to eliminate variability in native milk plasmin levels caused by the heating. Increasing severity of heat treatment, and hence whey protein denaturation, resulted in proportionately lower levels of primary proteolysis, as observed by urea-PAGE, indicating some alteration of the substrate on heating which renders peptide bonds less accessible to the enzyme. This effect was largely reversed by addition of 0.1 mM KlO(3). The proposed mechanism for this effect is binding of denatured whey protein to the surface of the casein micelle, with subsequent stearic blockage of the access of plasmin to susceptible peptide bonds. This work may suggest a mechanism for alteration of pathways of proteolysis previously noted in dairy products made from high-heat treated milk.