Following its discovery by Edwards and Hayes (1969), the propensity of 67Ga to accumulate preferentially in malignant tissues has been studied at diverse levels, not only for its importance in diagnostic imaging but also for its fundamental biological interest. The biodistribution of 67Ga citrate seems to be strongly influenced by the concentration of serum iron and the unsaturated iron-binding capacity (UIBC). Recently, it was found that the clearance of 67Ga from blood and the attainment of high tumor to normal soft-tissue concentration ratios are accelerated if the serum iron concentration is high. The present investigation was undertaken in order to study the relationships between the change of the concentration of serum iron and 67Ga accumulation in the tumor and soft-tissues in mice bearing Ehrlich's ascited tumor. The following results were obtained. (1) The uptake of 67Ga in the tumor and soft-tissues except bone was decreased compared with controls if the concentration of serum iron was elevated by Fesin (saccharated ferric oxide) administered prior the injection of 67Ga. (2) The urinary excretion of 67Ga was considerably increased if the concentration of serum iron was elevated by Fesin administered prior the injection of 67Ga. The results are consistent with the hypothesis that binding 67Ga to transferrin is very important to the accumulation of 67Ga in tumor and soft tissues.