A Western blotting method for the detection of whey milk proteins in commercial soymilks was applied to assess the food safety. Soy proteins and milk proteins were separated by SDS-PAGE in PhastSystem equipment. After the electrophoretic separation, immunodetection with anti-bovine alpha-lactalbumin and anti-bovine beta-lactoglobulin antisera was performed. Adulteration with bovine protein in percentages of 0.1% in soy protein can be detected. Western blotting of bovine alpha-lactalbumin and bovine beta-lactoglobulin was applied to detect adulteration by bovine milk proteins in different soymilks: powdered soymilk and soy infant formulas.