A method has been developed which appears to be reliable for differentiating between pasteurized milk and pasteurized milk containing a relatively small amount of reliquefied nonfat dry milk solids. This method is based upon the determination of the ferricyanide reducing value of the protein faction of milk. From the examination of 117 different brands of milk from different areas of the country, it was found that the protein reducing value of 4.07 mg potassium ferrocyanide per 100 ml of milk might be tentatively established as the maximum value for normal pasteurized milk. Any values above this indicate either (1) excessive heating of the milk during pasteurization, or (2) the presence of reliquefied nonfat dry milk solids or other forms of processed milk solids. If the conditions used in pasteurization are not known, determination of the undenatured whey protein nitrogen content of the milk will help in differentiating between these two possibilities.