A technique for labeling leukocytes with 99mTc using hexamethylpropylene-amineoxime (HMPAO) was evaluated in vitro. The labeling procedure resulted in a cell bound fraction of radioactivity of 56% after 99mTc incubation and 96% after 1 washing of cells. In the final cell suspension 84% of the radioactivity was attached to the polymorphonuclear (PMN) leukocytes constituting 94% of white cells. Only 5% was bound to residual red blood cells. The stability evaluated in autologous plasma showed a decline of cell bound activity from 96% to 84% over 3 h. The chemotactic function of PMN leukocytes was unaffected by the labeling procedure. These findings demonstrate that HMPAO, albeit cell unspecific, is efficient for labeling PMN leukocytes with 99mTc. The stability of the labeling procedure is high and the technique does not affect cell function. No other current 99mTc leukocyte labeling technique possesses all these qualities.