The proteolytic activity of both calf rennet and plasmin combined on bovine and caprine caseins was studied under various pH conditions by electrophoresis. Electrophoretic studies of the pH 4.6-insoluble fraction showed that both enzyme preparations (rennet and plasmin) hydrolyzed caseins, giving the typical breakdown products derived from individual caseins (CN): αs1-I-CN and β-I to β-III from αs1-CN and β-CN, respectively, by rennet action and γ-CNs from β-CN by plasmin action. These breakdown products were more or less evident in the hydrolysates depending on the pH conditions used, and they were subsequently hydrolyzed during the different hydrolysis times. Isolated γ-CNs were resistant to the rennet action, showing that although these breakdown products contain the chymosin-susceptible bonds of β-CN, presumably these bonds are inaccessible in γ-CNs. However, β-CN-derived products by the rennet action (i.e., β-I, etc.) were largely hydrolyzed by the plasmin action to yield peptides in the electrophoretic zone of γ-CNs. Bovine αs1-CN and para-κ-CN, the first proteolytic products from αs1-CN and κ-CN, respectively, by rennet, were degraded by plasmin, indicating that these breakdown products are susceptible to further proteolysis by plasmin in solution.