Bacterial DNA and immunostimulatory (i.s.) synthetic CpG‐oligodeoxynucleotides (ODN) act as adjuvants for Th1 responses and cytotoxic T cell responses to proteinaceous antigens. Dendritic cells (DC) can be referred to as “nature's adjuvant” since they display the unique capacity to sensitize naive T cells. Here, we demonstrate that bacterial DNA or i.s. CpG‐ODN cause simultaneous maturation of immature DC and activation of mature DC to produce cytokines. These events are associated with the acquisition of professional antigen‐presenting cell (APC) function. Unfractionated murine bone marrow‐derived DC and FACS^®‐fractionated MHC class II^low (termed immature DC) or MHC class II^high populations (termed mature DC) were stimulated with bacterial DNA or i.s. CpG‐ODN. Similar to lipopolysaccharide, i.s. CpG‐ODN caused up‐regulation of MHC class II, CD40 and CD86, but not CD80 on immature and mature DC. In parallel both DC subsets were activated to produce large amounts of IL‐12, IL‐6 and TNF‐α. CpG‐ODN‐activated DC displayed professional APC function in allogeneic mixed lymphocyte reaction and in staphylococcal enterotoxin B‐driven naive T cell responses. We interpret these findings to mean that bacterial DNA and i.s. CpG‐ODN cause maturation (first step) and activation (second step) of DC to bring about conversion of immature DC into professional APC.