A simplified and efficient procedure for 99mTc-HMPAO-labelling of leukocytes is described. For this purpose, the pH and concentration of the 99mTc-HMPAO preparation was modified. Leukocytes were isolated from a 20 mL mixture of patient blood, 5 mL ACD and 0.8 mL methylcellulose after 1 h sedimentation of erythrocytes and centrifugation (at 400 g) of the obtained plasma layer. Simultaneously, 99mTc-HMPAO was prepared (one single-dose kit for two patients) by adding 2.2 mL 99mTc-generator eluate and, after 10 min, 0.3 mL of phosphate buffer to lower the pH to 7. The isolated WBCs were then labelled by the addition of 1-1.2 mL of 99mTc-HMPAO solution and incubated for 20 min. The unbound tracer was then discarded, the labelled WBC washed and finally resuspended in autologous cell-free plasma. Leukocytes labelled by this procedure were used for scintigraphic localization of inflammatory lesions and abscesses in the gastro-intestinal tract. The labelling efficiency was 60 +/- 9%, with a separation yield of 55 +/- 11%.