Rats received a single intravenous injection with liposome-encapsulated dichloromethylene diphosphonate (C12MDP). This treatment resulted in the elimination of macrophages in spleen and liver within 2 days. Macrophages ingest the liposomes and are destroyed by the drug, which is released from the liposomes after disruption of the phospholipid bilayers under the influence of lysosomal phospholipases. Repopulation of macrophages in spleen and liver was studied at different time intervals after treatment. Macrophages in the liver (Kupffer cells) and red pulp macrophages in the spleen were the first cells to reappear, followed by marginal metallophilic macrophages and marginal-zone macrophages in the spleen. Different markers of the same cell did not reappear simultaneously. On the other hand, the same marker (recognized by the monoclonal antibody ED2) reappeared much more rapidly in the liver than in the spleen. The present results in the rat were different from those earlier obtained in the mouse. Red pulp macrophages were the first cells and marginal zone macrophages were the last cells to repopulate the spleen in both rodents after treatment with C12MDP liposomes. However, there was much more overlap in the repopulation kinetics of splenic macrophage subpopulations in the rat, when compared with the mouse.