Dichloromethylene diphosphonate (Cl2MDP) enclosed in liposomes, when injected intravenously, selectively eliminates all phagocytizing macrophages that are in direct contact with the blood circulation of the spleen and the liver. We examined whether Cl2MDP containing liposomes affect, in addition, rat monocytes and bone marrow macrophage precursors (BMMP's). In addition to Phosphatidylcholine-Cholesterol liposomes (PC liposomes), we also used mannosylated PC liposomes (PCMAN liposomes), which are reported to bind more efficiently to macrophages. Monocytes appeared to be affected 24 h after injections of 2 ml of Cl2MDP containing PC as well as PCMAN liposomes. In addition, almost no macrophages could be detected in one week cultures of blood leukocytes isolated from these animals; cultures from control animals contained +/- 50% macrophages. Bone marrow macrophage precursors did not appear to be affected.