Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry was employed to determine the changes in milk protein profile due to the action of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, the strains usually employed in yogurt production. Whereas the fermentation with the former shows the digestion of high molecular mass casein with the production of a low molecular mass peptide (3850 Da), the latter does not seem to exhibit any proteolytic activity. Proteolysis becomes relevant when milk is treated with a mixture of the two bacteria, reasonably due to symbiotic phenomena. The analysis of yogurt samples coming from industrial plants indicates increased activity of the two bacteria in the presence of ingredients containing sugars. Copyright 1999 John Wiley & Sons, Ltd.