Our initial experience with the clinical application of autologous leukocytes labeled in vitro with technetium 99m is reported. An injection of 3.6 X 10(8) leukocytes (83% of which were polymorphonuclear) labeled with 6.4 mCi 99mTc was administered to a patient with subcutaneous abscesses in order to evaluate the in vivo stability of the labeling and cellular function. Scintigraphic examinations demonstrated marked accumulations of radioactivity at the sites of inflammation, suggesting that 99mTc was efficiently bound to the cells and that the migration of polymorphonuclear leukocytes was not disturbed by the labeling procedure.