The activity of the synthetic immunomodulator LF 1695 on the efficiency of two effector cell populations--macrophages and blood platelets--involved in IgE-dependent cytotoxic processes against parasites, was evaluated. Oxygen metabolite production and anti-parasite cytotoxic properties of both macrophages and platelets were increased following LF 1695 treatment in vivo (in rat) or in vitro (in rat and in man). The phagocytic properties of rat peritoneal macrophages were also potentiated by their in vitro incubation with the drug. In addition to these effector functions, the lysosomal enzyme content and the migration ability of rat peritoneal macrophages were stimulated after incubation with LF 1695. In the presence of the drug, rat macrophages were also shown to produce increased level of IL-1--measured by the mitogen-induced proliferation of murine thymocytes--when compared to unstimulated phagocytes. Finally, the oral treatment of rats with LF 1695, in the course of an experimental infection with schistosome parasites, induced a higher degree of immune protection (80%) against a challenge infection than untreated, infected control rats (40%). These results bring evidence of a stimulatory role for LF 1695 on immune effector functions of cells participating to defense mechanisms against multicellular pathogens.