Protein spectra of blood serum were studied in 25 children suffering from phenlyketonuria (PKU) by disc electrophoresis in polyacrylamide gel. 14 children were examined in the course of being given a specialized diet on the basis of berlafen and 11 children were examined after they were switched over to common diet. PKU patients had an increased protein content in the zone of transferrins. After switching over to common diet the patients demonstrated the reduced protein content, increased amount of postalbumins of medium and low electrophoretic mobility, and transferrins as well.