Myelin was isolated from the brains of 16-day-old rabbits that had received intracerebral injections of [32P]orthophosphate 24 h earlier. The basic protein was extracted and examined by polyacrylamide gel electrophoresis both at low and at high pH. At low pH a single band corresponding to the basic protein contained all of the covalently bound protein radioactivity. At high pH, three bands of radioactivity corresponding to basic protein components 3, 5, and 7 were observed. Estimation of the specific radioactivities of the components in conjunction with their high pH electrophoretic mobilities indicated that components 3, 5, and 7 contained one, two, and three phosphate groups per molecule of protein, respectively.