[en] The anti-viral drug HPA-23 (ammonium 21-tungsto-9-antimonate) has been proposed for use in the combat against AIDS. The two elements tungsten (W) and antimony (Sb) in the molecule enable the intracellular localization and possible breakdown of the product to be studied using electron probe microanalysis methods. Such studies have been carried out after intravenous injection of different doses of HPA-23 in the rat followed by removal of the liver, kidney, thymus, spleen, bone marrow, and lung. HPA-23 was concentrated in the lysosomes and localized in the macrophages of different tissues (thymus, spleen, and bone marrow). The W/Sb ratio was identical in these macrophages. This localization is perhaps relevant to the mechanism of action of HPA-23