Capillary electrophoresis using a hydrophilically coated capillary and a low pH buffer containing urea has been used to follow the proteolytic action of plasmin and chymosin on isolated casein fractions, whole casein and individual milk samples selected on the basis of their genetic variants. Several of the main casein breakdown products were identified. These included, among others, ?1-casein (CN) A1, ?1-CN A2, ?1-CN B, ?1-CN C, ?2-CN A, ?1-CN A3, ?2-CN B, ?3-CN A and ?3-CN B, as well as proteose peptones, arising from the action of plasmin on the different genetic variants of ß-CN. as1-I-CN and as1-CN f(1-23) from as1-CN, and para-?-CN and caseinomacropeptide from ?-CN produced by chymosin action were also separated. The knowledge of their migration times provided information on the extent and origin of casein hydrolysis in both milk and cheese, as found in samples of proteolysed milk or fresh cheese.