Apoferritin, the iron-free protein from ferritin, has a molecular weight of 480,000 and a sedimentation coefficient of 17·6 s ( Rothen, 1944 ). It has now been degraded into subunits. Partial splitting of the native protein into a 2·1 s component is achieved by treatment with dilute alkali. This appears to be an all-or-none process. Native apoferritin is resistant to SDS,† urea and a number of other reagents, but can be split completely by SDS if first frozen, freeze-dried or dehydrated by alcohol. At SDS–protein ratios higher than 1 : 3 (w/w) the product is all in the form of a 2·5 s protein–SDS complex. At lower ratios both the 2·5 s and an intermediate 5·5 s component are present and also a component with average sedimentation coefficient 13·5 s which may be another intermediate, or a complex of SDS with undegraded apoferritin. Binding of SDS by freeze-dried apoferritin was measured by equilibrium dialysis. To obtain a homogeneous preparation of the 2·5 s component about 0·5 g SDS must be bound per gram of protein. The molecular weight of the complex was obtained both from measurements of S and D measured separately and from measurements of the ratio S : D from the Archibald approach-to-equilibrium method. The two methods gave reasonably consistent values. The molecular weights show a very marked concentration dependence, caused by an increase of D and a decrease of S with concentration. The values extrapolated to infinite dilution give a molecular weight of 38,000 to 41,000 for the complex and 25,000 to 27,000 for the protein subunit, allowing for the SDS bound. This would correspond to 18 to 19 subunits per molecule. A comparison with results obtained by other methods suggests that the number of subunits is actually 20.