The novel saponin based adjuvant Matrix-M? was recently used in a Phase I study of seasonal influenza in elderly. The present study is a pre-clinical evaluation of the efficacy and mode-of-action of Matrix-M? formulated influenza vaccine in mice. A manuscript on safety profile and immunogenicity in elderly humans is under preparation. We have previously shown that subcutaneous injections of Matrix-M?, without coformulated antigen, results in a dose-dependent recruitment of leukocytes to draining lymph nodes (dLNs). Herein we compared the mode of action of Matrix-M? with Alum, FCA and AS03 alone or formulated with influenza split virion antigen injected intramuscularly. The elicited responses in dLNs and spleen were investigated 48h later. Matrix-M? was particularly efficient in activation of central innate immune cells such as neutrophils, DCs and macrophages compared to the other adjuvants analyzed. Moreover, the adjuvant influence on the recall immune response to influenza antigen was studied by in vitro re-stimulation of splenocytes from mice immunized with influenza antigen adjuvanted with Matrix-M?, Alum or AS03. Splenocytes from mice immunized with influenza antigen and Matrix-M? produced both Th1 and Th2 cytokines upon re-stimulation. This response was significantly stronger than that induced by the other adjuvants studied. Interestingly, increased levels of the neutrophil chemoattractant KC were produced by antigen stimulated splenocytes from mice immunized with Matrix-M? adjuvanted vaccine, which is in agreement with the increase of neutrophils into dLNs and spleen after Matrix-M? injection. Furthermore, influenza antigen adjuvanted with Matrix-M? induced significantly higher antigen-specific IgG1 and IgG2a responses compared to antigen alone. In conclusion, adjuvant Matrix-M? activates the innate immune system without antigen present. This activation may explain the enhanced immunity to influenza seen with Matrix-M? adjuvant. Despite this potent immune activation mediated by Matrix-M?, GLP-toxicity studies and clinical data suggest that Matrix-M? adjuvant has a mild to moderate safety profile.