The authors employed (99m)Tc as a radioisotopic label to study the migratory patterns of syngeneic and xenogeneic lymphocytes and erythrocytes in mice. Maximum splenic and hepatic localization of lymphocytes and erythrocytes was attained 1 to 2 hr following intravenous injection. The mean liver/spleen ratio was 6.2 with syngeneic murine thymocytes and 14.0 with human peripheral blood lymphocytes. When syngeneic murine erythrocytes were administered intravenously approximately equal numbers of cells localized in the liver and spleen. In contrast, when xenogeneic sheep RBC were given, there was markedly increased hepatic localization as reflected by a mean liver/spleen ratio of 99.6. There was a linear relationship between the number of lymphocytes or erythrocytes injected and gamma counts recorded in the liver and spleen. Technetium 99m also could be employed as a label to study the distribution of Sarcoma I cells in syngeneic A/J mice. The greatest number of tumor cells localized in the liver and lungs in a ratio approximating unity and this was maximal at 4 hr. Significant numbers of counts also were detected in the kidneys, less in the spleen, and relatively few in the peripheral blood. These studies are the first to show that (99m)Tc can be used as a label to follow the organ distribution of both normal and neoplastic nucleated cells. The high specific activity, rapid labeling time, lack of release from dead and injured nucleated cells, and ready availability of the radionuclide are significant advantages which suggest that (99m)Tc may have wide applicability in both experimental and clinical studies of cell migration.