In order to characterize the role of transferrin receptors (Tf. r.) in the process of ⁶⁷Ga uptake, we investigated the relationship between ⁶⁷Ga uptake and the number of Tf. r. in cultured cells (NRK-49F cells, NRK-52E cells). NRK-49F cells derived from fibroblast of normal rat kidney and NRK-52E from epithelial cell of normal rat kidney were cultured in Dulbecco's MEM medium. ⁶⁷Ga uptake and cell numbers were counted by well type Scintilation counter and by the Coultor counter, respectively. The number of Tf. r. was analized by Scatchard plot. These two kinds of cell lines had two phases of cell proliferation, one was a proliferate phase and the other was a quiescent phase. In a proliferate phase, cells having higher proliferative activity had much more transferrin receptors than that in a quiescent phase. The amount of ⁶⁷Ga accumulated in these cells in proliferate phase was also much more than that in quiescent phase, although the affinity constant of Tf. r. for transferrin was same value between these two phases. These results suggest that the ability of ⁶⁷Ga uptake is well correlated with the number of Tf. r. per cell and with the activity of cell proliferation.