Studies of the in vivo kinetics of granulocytes labeled in vitro with indium-111 were carried out in 10 normal subjects. The granulocyte suspension was prepared with a Ficoll-Hypaque density gradient and labeled with indium-111-oxine. No elution or reutilization of the radioisotope was demonstrable in vitro. The average intravascular recovery of indium-labeled granulocytes was 30% +/- 6, and the t 1/2 was 5.0 +/- 1.6 hr. Normal in vitro function of these cells was demonstrated by bacterial killing and chemotaxis assays. Because indium-111 decays by gamma emission, the fate of in vivo labeled granulocytes can be followed with scintigraphic techniques. Images obtained indicated normal uptake of activity in the liver and spleen. Effective in vivo function of indium-labeled granulocytes was demonstrated in four patients by the localization of radioactivity at sites of inflammation or abscess. Although the intravascular recovery with this method is lower than that reported for some other radioisotope methods, the comparison of abnormal findings with normal values by this technique is probably valid. Indium-labeled granulocytes should prove useful in the study of granulocyte collection, transfusion, histocompatibility, and storage.