Abstract Erythrocyte labeling with 51Cr or 99 m Tc is a well-established procedure in nuclear medicine. The labeled cells are useful in the measurement of red cell mass, in erythrocyte survival studies, and in the localization of erythrocyte pools. Attempts to label the leukocyte with the same radionuclides have not been as successful. This study reports on various laboratory parameters of labeling leukocytes with 99 m Tc. Microgram quantities of stannous chloride are used in the labeling process to reduce 99 m Tc-pertechnetate to a lower valence state. Other reducing agents such as ferrous ion and penicillamine are not effective. Pertechnetate itself is only loosely bound to the leukocyte. An electrophoretic technique is used to determine labeling yield. Differences in labeling efficiency between the cells from normal donors and the cells from chronic myelocytic leukemia (CML) donors have been detected. Leukocytes from normal donors generally label with greater efficiency than leukocytes from CML donors, but subject age is probably a contributing factor, with cells from young normal donors labeling more like cells from CML donors. The leukocyte and erythrocyte can be labeled under identical conditions in about the same yield. Labeled leukocytes retain good viability and phagocytic capability. Although autoradiographic studies are not completely conclusive because of the poor radiation characteristics of 99 m Tc, it appears possible that the -Sn- 99 m Tc label is on the surface of the cell and is not intracellular. The labeling process is being adapted to an aseptic process for future experimentation in animals and in human subjects.