Capillary electrophoresis (CE) and mass spectrometry have been used to verify the formation of lactosylated casein variants. CE was performed in a hydrophilically coated capillary at low pH and in the presence of urea. Lactosylated alpha-casein, beta-casein, and kappa-casein migrate shortly after the unmodified proteins. Evidence for casein lactosylation was obtained by electrospray ionization-mass spectrometry (ESI-MS). Lactosylated beta-casein can be monitored not only in milk and milk powders but also in the complex protein mixture of processed cheese. The formation of lactosylated beta-casein A1 and A2 during heating of processed cheese was found to correlate with the furosine content. Consequently, CE of casein may be a possible method for directly assessing the extent of the early Maillard reaction in dairy products.