The radiolabeling of platelets has been studied for many years, both with megakaryocytes labeled in vivo and with direct platelet labels in vitro. The major aim of this work has been to evaluate platelet interactions in vivo. This has been made possible with indium-111-labeled platelets. The radionuclide is easily imaged and can be incorporated into platelets with ease. Unfortunately, the lipophilic complex used is not platelet-specific and must be exposed only to the isolated cell population for specific labeling. This requires isolation of platelets from whole blood followed by one of many variations of differential centrifugation, buffer washes, and resuspension techniques that have been reported. The major differences in these techniques are the resuspension media, the incubation time, and the ligand used. These variations are discussed with emphasis on known platelet characteristics and specific responses to these modifications.