Background: The efficacy of homeopathic medicines for maintaining human health and treating disease has been extensively examined in clinical trials. However, there is a paucity of preclinical evaluations of the effects of homeopathic medicinal preparations on cellular signaling pathways relevant to the applications of these preparations.Materials and methods: In this study, the immune-modulatory effects of Phase 6 (for the stimulation of the nonspecific defense system) and Flu Terminator® (for influenza and viral diseases) (Be Well Homeopathics Inc. Miami, FL), two homeopathic preparations developed for the purpose, were evaluated in normal human leukocyte cultures in vitro.Results:Both Phase 6 and Flu Terminator stimulated the production of pro-and anti-inflammatory cytokines by human leukocytes, although higher doses often produced a weaker response than lower doses. The carrier solvent (20% ethanol) failed to elicit any cytokine synthesis.Conclusions:The results of the in vitro studies suggested that ultralow concentrations of ingredients in Phase 6 and Flu Terminator were capable of eliciting a human immune response.