We studied the tumor uptake of [⁶⁷Ga]citrate, [⁵⁹Fe]citrate, and ¹²⁵I-labeled transferrin (TF) by the in vitro growth form of EMT-6, a sarcoma-like mammary tumor of BALB/c mice. In analyzing the binding experiments, we developed a new mathematical model based on a formulation originally used to exprell the interaction of hormones with specific tissue receptors. The uptake of both carrier-free ⁶⁷Ga and ⁵⁹Fe by tumor cells was mediated by kinetically identical TF receptors. We also studied terlc acid extracts of the stroma of EMT-6 tumors grown both in vivo and in vitro. Chromatography of these extracts on Sephacryl S-200 SF demonstrated that the cellular stroma contained specific TF-binding macromolecules. On the basis of these findings, we proposed the “transferrin receptor hypothesis” for the mechanism of ⁶⁷Ga uptake by tumors. According to this view, a tumor-alloclated TF receptor is the functional unit responsible for the affinity of gallium for certain neoplasms. This receptor was also active in the uptake of iron by tumors.