Bovine milk whey proteins were treated with porcine stomach pepsin, bovine chymosin, human uropepsin, and gastric juice from rat stomach at various pH values. Although α-lactalbumin and bovine serum albumin were hydrolyzed by these proteases, β-lactoglobulin (β-LG) was not hydrolyzed by any pepsin samples at any pH in vitro. Heat-denatured whey proteins, including heated β-LG, were easily hydrolyzed by pepsins. Native β-LG injected into the stomach of a rat was not digested in stomach in vivo, whereas heat-denatured β-LG was digested in the stomach of a rat. Native β-LG as well as the heated sample was hydrolyzed by both commercially available porcine pancreatin and the rat pancreatic juice collected from rat intestine in vitro. Native β-LG injected into the stomach was digested in the intestine in vivo as well as heat-denatured β-LG. The body weights of rats fed heated β-LG increased more rapidly than those of rats fed native β-LG. Keywords: β-Lactoglobulin; milk whey protein; pepsin; pancreatic enzymes;...