To develop a radiopharmaceutical for apoptosis imaging, Annexin B1, a new Ca2+-dependent phosphatidylserine (PS)-binding protein, was directly radiolabeled with Tc-99m. This procedure yields up to 96% of radiochemical purity and higher radiolabeling efficiency. The preparation has been found to be sufficiently stable in vitro. Binding assay with human activated platelets indicated that Tc-99m-Annexin B1 retained its PS binding activity. Biodistribution in mice revealed that Tc-99m-Annexin B1 rapidly cleared from the blood and predominantly accumulated in the kidney. The increase in hepatic uptake in anti-Fas antibody treated mice correlated to histologic evidence of fulminant hepatic apoptosis. These data suggest that Tc-99m-Annexin B1 can be used as a novel radiotracer to detect apoptosis in vivo. (c) 2005 Elsevier Inc. All rights reserved.