The importance of macrophages as effector cells in cellular reactions of immunity and hypersensitivity is well established (8). Macrophages activated as a result of specific immunological events exhibit increased microbicidal activity not only for the primary organism used for immunization purposes but also against unrelated intracellular pathogens (7). It is believed that the acquisition of this bactericidal or bacteriostatic activity is conferred on macrophages by materials released during the interaction of specifically immune lymphocytes with the antigen (Fowles et al., 1973; Godal et al., 1971; Jones and Youmans, 1973; Krahenbuhl and Remington, 1971; Mackaness, 1971; Patterson and Youmans, 1970). The antimicrobial activity in these studies refers to the intracellular inhibition and/or killing of the organisms; no attempts were made to assess the effect of activated macrophages on the extracellularly residing organisms. In an earlier study with the pathogenic yeastlike fungus Cryptococcus neoformans, we observed that freshly collected peritoneal macrophages from mice pretreated with a variety of nonspecific agents or from specifically immune mice possessed the capacity to exert anti-cryptococcal effect both on intracellular yeasts as well as those present in the extracellular milieu (Sethi et al., 1971; Sethi and Pelster, in press). Our results suggest that soluble product(s) released from mouse macrophages immune to Listeria monocytogenes can exert antilisterial activity under in vitro conditions.