Dry weight, total basic protein and myelin basic protein (MBP) in human corpus callosum tissue were found to be significantly reduced with age, confirming other reports. The heterogeneous patterns of the MBP samples were investigated and could be classified into three groups: a normal one with peak 1 larger than peak 3; an old-age pattern with peak 1 smaller than peak 3; and a pathological pattern with peak 1 greatly reduced. Because peak 1 is the only one which is significantly reduced with age, it is suggested that future investigations could be simplified. The physiological and pathophysiological role of MBP heterogeneity is discussed.