More than hundreds of different bioactive peptides are located in the primary structure of milk serum proteins. β-Lactoglobulin (β-LG), α-lactalbumin (α-LA), serum albumin (SA), lactoferrin and immunoglobulins belong to the bioactive peptides precursors . For the study and use of bioactive peptides, it is necessary at the first stage to select their homogeneous precursors. The purpose of the study was to isolate homogeneous precursors of bioactive peptides from serum proteins by preparative electrophoresis in a polyacrylamide gel. Homogeneity of protein fractions at different stages of their selection was analyzed by analytical disc-electrophoresis (Davis system) for acidic neutral proteins. The protein concentration was determined by spectrophotometer (SF-46, λ = 280 nm). In this case, previously established absorption coefficients (D^1%_1cm) were used: 12.3 for total serum protein, 9.6 for β-LG, 20.9 for α-LA, 6.9 for SA, 9.91 for lactoferrin and 13.6 for immunoglobulins. Homogeneous fractions for identification of milk serum proteins were isolated by double gel-filtration on Sephadex G-100 (fine). Preparative electrophoresis was performed on a modified apparatus of Stadier type. Milk serum was isolated after precipitation of caseins at the isoelectric point. From the 3 variants of the anode electrophoretic systems (disc-electrophoresis with sodium dodecyl sulfate, disc-electrophoresis in the native conditions, electrophoresis in the presence of uric acid), disc-electrophoresis in the native conditions was selected due to its efficiency and accessibility. The isolation of homogeneous fractions included preparative disc-electrophoresis, identification of fractions, their extraction and drying. Based on the results of 5 preparative electrophoresis, it was shown that the total protein yield was 67.9% (P < 0.05). Among fractions, the immunoglobulins had the lowest yield (<60%). The β-LG, α-LA, and SA fractions had a yield close to the total protein one.