This study developed a modified pretinning method for labeling erythrocytes with technetium-99m in vitro using a kit which contains stannous chloride stabilized with gentisic acid (GA) in lyophilized form. This GA-kit is a sterile closed system that does not require centrifugation, washing and separation of the RBCs. The kit is simple to use and labeling takes 30 min with a typical labeling efficiency of 97.3%±1.2% for 60 determinations. Imaging studies were performed in rabbit using GA-kit in vitro labeling procedure and PYP-kit in vivo labeling. It shows that the heart-to-background ratio with GA in vitro labeling were significantly higher than with in vivo labeling. Leukocytes labeling with technetium-99m using the same GA kit also was studied. The labeling efficiency was 20–41%. Thus, the GA kit is versatile and cost-effective.