Abstract: Bisphosphonates inhibit the activity of osteoclasts and conceivably also macrophage activity. Administered in hypoosmotic solution, clodronate (dichloromethylene bisphosphonate) forms complexes with the iron of haemolysed erythrocytes and subsequently accumulates in splenic and hepatic macrophages. Pamidronate (3-amino-1-hydroxypropylidene bisphosphonate) also accumulates in the spleen and liver of mice and rats even when injected in isoosmotic solution. In the present study, the effects in mice of uncomplexed clodronate, clodronate-iron complex, clodronate-liposomes, and pamidronate on splenic and hepatic macrophages in vivo were studied by an enzyme-histochemical method. Intracellular free clodronate released in the cells after phagocytosis of clodronate-liposomes virtually eliminated all macrophages from spleen, and also Kuppfer cells from liver. Clodronate given in hypoosmotic vehicle had no effect on these cells, although it was taken up by them. Clodronate in isoosmotic vehicle, considered as uncomplexed clodronate, neither accumulates in nor affects the macrophages. Pamidronate, however, decreased the number of the cells also after isoosmotic injection. Thus, the capability of bisphosphonates to sequestrate intracellular calcium may explain their effects on macrophages.