Plasmacytoid dendritic cells (PDCs) are a subset of dendritic cells present in human blood and inflamed lymph nodes. Here we show that blood PDCs, when stimulated with influenza virus and CD40L in vitro, undergo a maturation process characterized by up-regulation of major histocompatibility complex proteins and adhesion and costimulatory molecules. In addition, PDCs down-regulate CXCR3 and L-selectin, which mediate migration and homing of these cells into the lymph node. Mature PDCs efficiently stimulate T cells and drive a potent TH1 polarization in vitro, which is mediated by the synergistic effect of interleukin 12 and type 1 interferon. In vivo, mature PDCs are found in secondary lymphoid organs, where they represent the principal source of type 1 interferon during inflammation. Thus, PDCs probably participate in antiviral and pro-inflammatory responses, rather than in TH2 polarization and tolerance induction.