Tc-99m-Hexamethyl-propylenamine-oxime (Tc-99m-HMPAO)-labelled leukocytes have been used in standard diagnostic procedures for the detection of infection and inflammation. Although some investigators have already pointed out that labelling of leukocytes with 99mTc-HMPAO has detrimental effects on the cells, still very little is known regarding the effects of ionizing radiation on lymphocyte function. The effects of Tc-99m-HMPAO-labelling on lymphocyte adhesion, proliferation, mitotic index, migration and apoptosis were evaluated. The lymphoblastoid cell line NC-NC was used as the lymphocyte population. 99 Tc-HMPAO-labelling decreased cell adhesion, proliferation, mitotic index and motility, whereas it induced apoptosis and cell-cycle arrest. The rate of decrease in cell proliferation was up to 70% (P < 0.001) by day 4 after labelling. Tc-99m-HMPAO-labelling led a 35% decrease (P < 0.001) in adhesion ability of the cells on fibronectin at 16h. Using the Boyden chamber motility assay, it was shown that both spontaneous and monocyte chemotactic protein (MCP-1)-induced lymphocyte motility were strongly reduced by Tc-99m-HMPAO-labelling. The decrease in motility was approximately five-fold (P < 0.05). In addition, a 12-fold increase (P < 0.05) was observed in apoptosis of the Tc-99m-HMPAO-treated cells compared with control cells. Besides, it was shown that cell-cycle arrest was induced starting from the 3rd day after treatment with Tc-99m-HMPAO. Our observations indicate that Tc-99m-HMPAO-labelling has damaging effects on lymphocyte function including cell adhesion, proliferation, mitotic index, motility and cell cycle under ill vitro conditions. (c) 2007 Elsevier B.V. All rights reserved.