Equilibrium-dialysis experiments with 59Fe-labelled Fe(III) chelate solutions show that ferritin is capable of binding a limited number of Fe(III) atoms. Some of this Fe(III) is readily removed, but up to about 200 Fe(III) atoms/molecule remain bound after extensive washing. Some exchange of labelled Fe(III) with endogenous unlabelled ferritin Fe occurs during prolonged dialysis against 59Fe(III)-citrate, but there is a net binding of Fe(III). Bound Fe(III) resembles endogenous Fe(III) in several respects. It appears to be attached to the micelle and not to the protein component of ferritin. Although the physiological mechanism of Fe incorporation into ferritin is unknown, our experiments suggest the possibility that some iron finds its way into ferritin as Fe(III) chelate.