The aim of this study was to clarify the mechanism of adjuvant action induced by negatively charged liposomes composed of phosphatidylserine (PS-liposomes). Coculture of mouse splenic adherent cells with ovalbumin (OVA)-specific Th1 clone (42-6A) cells in the presence of PS-liposomes encapsulating OVA resulted in high levels of IL-12 and IFN-gamma productions compared with those of positively charged or neutral liposomes. IL-12 and IFN-gamma productions were dose-dependent on antigens in PS-liposomes. The expression of CD40L on 42-6A cells was enhanced by treatment with PS-liposomes encapsulating OVA. IL-12 production, but not IFN-gamma, was completely inhibited by the addition of anti-CD40L mAb. These results indicate that PS-liposomes encapsulating antigens could enhance antigen-dependent IL-12 production through the interaction between CD40 on macrophage/dendritic cells and CD40L on 42-6A cells.