Stannous chloride was evaluated as a stannous ion source for the in vivo labeling of red blood cells(RBC) with Tc-99m. In this study, the labeling of RBC with Tc-99m was performed by two successive intravenous administrations of stannous chloride and Tc-99m-pertechnetate, and the optimal dose of stannous chloride and the optimal time interval between the two injections were evaluated. The labeling efficiency for this procedure was also evaluated as a function of time after the pertechnetate injection. The results of our investigation revealed that the maximal in vivo RBC labeling (86%) can be obtained at 15 min after the pertechnetate injection with an i.v. dose of 12.7 ..mu..g/kg of stannous chloride followed 15 min later by an i.v. injection of Tc-99m-pertechnetate. In conclusion, stannous chloride was found to be an excellent stannous ion source for the in vivo labeling of RBC with Tc-99m.