The retention values for 42K, 86Rb and 134Cs in the tissues and blood were quite similar to those for 201Tl, but were very different from those for 22Na. In an experiment for subcellular fractionation of tumors, most of these nuclides were localized in the supernatant fraction, with small amounts in other fractions. The concentration ratios for these nuclides in each fraction were approximately constant regardless of the time after administration. Radioactive alkaline metals in the supernatant fraction of the tumor homogenate existed mostly as free ions and were bound to protein in other fractions of tumor tissue. These results were essentially the same as those for 201Tl. Ouabain suppression studies indicated that 201Tl is taken up into the tumor cells partly through Na+, K+-ATPase of their membranes. Ionic radii of alkaline metals and thallium were related to their blood and tumor retention values. This relationship suggested that monovalent cations whose ionic radii exceed 0.133 nm, and which exist as free ions in the tissue fluids, behave like the potassium ion. Potassium and K analogs (Tl, Rb, Cs) are avidly taken up into viable tumor cells whose Na+, K+-ATPase activity is elevated. Therefore, suitable radionuclides of K and K analogs can be excellent agents for visualization of viable tumor tissues.