Ehrlichia risticii incubated with mouse peritoneal macrophages elicited with thioglycolate broth survived and replicated, thereby allowing examination of the effects of several immunopotentiating agents. Treatment of the macrophages with recombinant murine gamma interferon (rMuIFN-gamma) in vitro at 1 day before or 3 h after infection made the macrophages resistant to infection with E. risticii, and macrophages treated with rMuIFN-gamma at 1 to 3 days after infection developed the capacity to eradicate intracellular E. risticii. Similar effects were seen with macrophages treated with the Ca2+ ionophore A23187 before or after E. risticii infection in vitro. Concanavalin A treatment before or 3 h after infection caused the macrophages to become resistant to infection with E. risticii but could confer neither ehrlichiacidal nor ehrlichiastic activity to them once infection had been established for more than 1 day. Bacterial products such as lipopolysaccharide and muramyl dipeptide were less or not at all effective, respectively, in conferring antiehrlichial activity to macrophages. Finally, protein kinase C activator, phorbol myristate acetate, and recombinant tumor necrosis factor did not induce any antiehrlichial activity in macrophages when the macrophages were treated either before or after infection.