The proteolysis of highly purified samples of αs1-, αs2-, β-and κ-caseins by porcine plasmin and by bovine plasminogen with urokinase has been examined principally by gel electrophoresis. The resulting peptide band patterns were compared with those of total proteose-peptone (TPP) samples prepared from fresh and stored raw and pasteurized milk, and also with those obtained during the natural course of proteolysis by indigenous enzymes in milk during storage. TPP was found to contain at least 38 components detectable by a single electrophoresis run. Apart from residual traces of whey proteins and intact caseins nearly all of these components were fragments of caseins produced by indigenous plasmin, with products from the breakdown of αs1- and β-casein predominating. Over 90 % of TPP has been accounted for in this way. A fragment consisting of residues 29–105 of β-casein was isolated and characterized from both stored milk and from plasmin digests of β-casein. This fragment was a relatively major product of the natural proteolysis occurring during storage of milk, but contrary to a report in the literature it was not the same as proteose-peptone component 8-slow. Since many of the components of TPP resulted from proteolysis, the composition of TPP was found to vary according to the time and temperature of storage of the milk from which it was prepared. Thus, while the proteose-peptone fraction of milk can easily be defined operationally it cannot be rigorously defined in terms of its composition unless the history of the milk is also defined.