Suboptimal labeling of red blood cells with Tc-99m, using the modified in vivo technique, was observed in three patients in whom labeling was attempted following IV administration of iodinated contrast media in the previous 24 hours. The mechanism is not understood, but it may represent a change in either stannous ion distribution or redox potential. It is recommended that studies employing Tc-99m RBC labeling using this technique be performed prior to administration of iodinated contrast media. This effect should be kept in mind in gastrointestinal bleeding studies performed using Tc-99m RBC, as well as in cardiac imaging studies.