A new method, based on the pretreatment of leukocytes with glucoheptonate prior to treating with reduced 99mTc, has been developed for the preparation of 99mTc labelled leukocytes. The leukocytes labelled with a 99mTc concentration (5.59%/g tissue) similar to that of 111In-leukocytes (6.27%/g tissue), in the experimental abscess were in rat thigh. Concentration of 99mTc-leukocytes in blood at 24 h was only about 35% as compared to that of 111In-leukocytes. Biodistribution in the rat organs was similar in both cases, except in the liver where 99mTc-leukocytes exhibited about 4-fold greater concentration. Images of experimental abscess in rat by using 99mTc-leukocytes were comparable to those obtained with 111In-leukocytes.