Using two types of cultured cells, fibroblasts (NRK-49F) and epidermal Cells (NRK-52E) of a normal rat kidney, the number of transferrin (Tf) receptors and the amount of Ga-67 uptake was determined in both proliferative and quiescent phases. The following results were obtained: 1. There was almost no uptake of H-3 thymidine in the quiescent phase in both NRK-49F cells and NRK-52E cells. 2. Ga-67 uptake in the quiescent phase decreased to about 28% of the value in the proliferative phase in NRK-49F cells and to about 23% in NRK-52E cells. 3. The number of Tf receptors in the quiescent phase decreased to about 10% of that in proliferative phase in both NRK-49F and NRK-52E cells. 4. Using the immunoperoxidase method with Nu-TfR1 (anti-Tf receptor monoclonal antibody), the antigen, Tf receptor, was confirmed in the region corresponding to the cell membrane in the proliferative and quiescent phases. In addition, highly positive findings were observed frequently during cytokinesis in the proliferative phase. These results suggest a correlation between the number of Tf receptors and Ga-67 uptake in the proliferative and quiescent phases it indicates that Ga-67 uptake into the cells depends on the number of Tf receptors.