Equilibrium dialysis was used to examine the binding of Ga-67 to horse spleen ferritin (HFE), and the ability of this protein to remove Ga-67 originally bound to human transferrin (TF). Seventy hours were required for the HFE to bind 70% of the activity. When HFE was placed in competition with preformed TF-Ga-67 complex, little nuclide was translocated to HFE. Upon the addition of compounds of low molecular weight that occur intracellularly, this transfer was dramatically enhanced. In the presence of 1 mM adenosine triphosphate (ATP), the most effective mediator examined, the final distribution was 17% bound to TF and 62% to HFE, with 16% not protein-bound. In the absence of any mediator, the same distribution was 84, 6, and 3%. Control experiments with ATP showed that little radionuclide was transferred from TF to albumin. These results add support to the previous suggestions of the potential role of ferritin in Ga-67 localization.