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The flavor, physical properties, and nutritive value of food products, such as dried milk, are often affected markedly by the so-called Maillard reaction between the carbonyl groups of sugars and the amino groups of protein and/or amino acids. Animal evaluation of the damage done to the nutritive value of the protein in such products due to the Maillard reaction is costly in both time and animals. Hence, it is difficult to assess the relative influence of processing variables and storage on such products. Measurement of the microbiologically available lysine liberated from resuspended whey powders by digestion with crystalline trypsin affords a satisfactory prediction of animal growth response on the powders. The method is also applicable to milk powders, milk concentrates, and protein fractions derived from whey powders. Digestion with a series of digestive enzymes (pepsin, trypsin, and chymotrypsin) does not give a good prediction. Processing variables-in particular, drying methods and storage temperatures-affect the nutritive value of whey powders measured in vivo and by the enzymatic test.