A rapid and simple method for labeling mammalian nucleated cells with (99m)Tc is described. A wide variety of tissue culture cells of mouse and human origin as well as lymphocytes isolated from peripheral blood were labeled without appreciable loss of viability. Optimum labeling was obtained by incubating viable cells suspended in a balanced salt solution with 10 mCi (99m)Tc and 100 μg Na2CrO4 for 15 min at 37° C followed by reduction with 0.2% SnCl2 solution. Serum protein interfered with the labeling process. 10,000 viable cells gave between 1.2 and 6 x 105 cpm immediately after labeling. Mammalian nucleated cells retained approximately 90% of the bound radioisotope when injured or killed, and that which is released does not appear to be in a reusable form. Labeling with (99m)Tc had no consistent effect on the cells' ability to synthesize DNA or proteins.