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We have compared the size and shape of ferritins from human heart, rat heart, and skeletal muscle with those of rat liver and horse spleen ferritins, using sedimentation and gel filtration techniques. The electrophoretically "fast" form of heart ferritin was partially separated from both the "slow" heart form and from rat liver ferritin by gel filtration (Stokes radii 72 A versus 69 and 68.5 A). Sedimentation velocity after iron removal showed an 18.5 S boundary even for mixtures of the two heart species, versus 17.3 S for rat liver and horse spleen apoferritins. Holoferritins gave a broad boundary, with coefficients from 66-80 S depending on iron content. Variable amounts of disaggregated ferritin (2.6 S) were also present in the heart ferritin preparations. Removal of iron significantly increased the electrophoretic migration of the fast but not the slow heart ferritin species. By sedimentation equilibrium, the molecular weights of all apoferritins save the fast heart form were about 490,000; that for the latter was near 750,000. Since electrophoresis revealed no major differences in subunit size, it is concluded that the larger, more asymmetric form of muscle ferritin contains 34 to 38 rather than 24 subunits.