In this study we present a new method for the elimination of mononuclear phagocytic cells from cell suspensions. By making use of liposome-encapsulated dichloromethylene diphosphonate we were able to effectively remove macrophages from spleen cell suspensions. This effect was not observed when using the free drug or control (PBS) liposomes. The use of this procedure has no effect on other cell types, as measured by growth, protein production, antigen presentation and antigen specific T cell proliferation, though PBS liposomes in very high doses were able to inhibit antigen presentation. The finding that lymphocytes are not affected by the liposome encapsulated drug suggests that the observed loss of lymphocytes in vivo, after intravenous dichloromethylene diphosphonate liposome treatment, may be due to damage inflicted by lysosomal enzymes released from dying macrophages. This method permits the removal of both macrophages and monocytes from heterogenous cell populations (i.e., blood, lymphoid tissue suspension) in vitro with a very high rate of reliability. With the concentrations and incubation time used, no negative effects on other cell types were observed.