Electrophoretic mobilities of ferritins from horse heart, liver and spleen were compared in the pH range 3.5-10. Electrophoretic titrations and continuous buffer electrophoreses were used. The order of anionic mobilities was heart > liver > spleen at alkaline pH and this order was reversed in the neutral to acidic range. This order of mobilities, and the intermediate behavior of liver in respect to the other two ferritins correlate with the known subunit composition of the three ferritins, and strongly support the idea of different amino acid residues being exposed on the protein shell surfaces. From the analyses of differential mobility around the pK values of the various ionizable groups it was concluded that heart and spleen ferritins have a very similar number of acidic amino acid residues on their surfaces, whereas they differ in basic residues. Heart seems to have about 15% more Lys and Arg, and twice as many His as spleen.