A new preparative method is described for the separation and direct electrophoretic comparison of the distribution profile of radioisotopically labeled cells. The cell populations to be compared are labeled separately with different radioisotopes (e.g., 51Cr and 99mTc) and mixed. The cell mixture is subjected to density gradient electrophoresis and the distribution of each radioisotope in the collected fractions is ascertained by differential gamma counting. In the case of uniform label uptake by the cells in each distribution, the radioactivity counts represent cell frequency. Nonuniform labeling allows useful comparisons only in terms of relative mobility distribution. Model experiments performed with 51Cr- and 99mTc-labeled mouse thymocytes and spleen cells are in general agreement with mobility distribution results obtained previously by free flow electrophoresis.