Preliminary experience with 99mTc labeling of isolated polymorphonuclear leukocytes is reported. Leukocytes efficiently purified from 100 ml peripheral blood were labeled in vitro with 99mTc after pretreatment with small amounts of a stannous agent to reduce pertechnetate. A labeling yield of 1.3 mCi equivalent to 12.5% of the added dose of 99mTc and a cell binding efficiency of more than 90% was achieved under sterile conditions. Viability assays showed no influence of the labeling procedure on cell function. The technique may prove a valuable alternative for assessment of local leukocyte accumulation by scintigraphy.