The method described by Weisner et al. for stabilizing 99Tcm-hexamethylpropylene amine oxime (99Tcm-HMPAO) with cobalt chloride hexahydrate solution is the most promising developed to date. The aim of our work was to study the behaviour of cobalt during the labelling of leukocytes with 99Tcm-HMPAO stabilized in vitro using this method. Three parallel labellings were carried out using six blood samples taken from polycythaemic patients. The first set of labellings was performed using 99Tcm-HMPAO, the second set using 99Tcm-HMPAO stabilized with cobalt chloride hexahydrate solution "spiked' with 57Co-chloride, and the third set using 57Co-chloride solution alone. Measurements of radioactivity content were made on the leukocyte pellets after washing at five time points post-labelling (t = 0, 30 min, 1 h, 2 h and 4 h). The data show that leukocytes do not retain cobalt during labelling with stabilized 99Tcm-HMPAO. Studies carried out in parallel demonstrated that the presence of cobalt in the cell-labelling medium had no effect on cell viability.