Lactosylation in stored milk powder was quantified by multiple reaction monitoring (MRM), a mass spectrometry-based quantification method. The MRM method was developed from a knowledge of peptide fragmentation. The neutral losses of 162Da (cleavage of galactose) and 216Da (the formation of furylium ion) which were representative of lactosylated peptides were specifically selected as MRM transitions. Quantification of lactosylated protein was based on the peak areas of these fragmentation ions. The MRM results showed an increase in peak areas of the two transition fragments from tryptic digests of whey proteins in stored milk protein concentrate powder. A good correlation between the MRM and furosine results indicated that MRM based on tryptic digests of whole products was a feasible method for quantification of modified milk proteins.