Dendritic cells (DC) can be generated from mouse bone marrow (BM) in the presence of granulocyte‐macrophage colony‐stimulating factor (GM‐CSF). Bacterial stimuli such as endotoxin / lipopolysaccharide (LPS) can induce their final maturation. When BM‐DC cultures were treated at day 6 or later with LPS, this final maturation was induced in vitro within 24 h. Such mature DC exhibited high levels of surface MHC II molecules and potent T cell sensitizing, but reduced endocytosis capacity. In contrast, immature DC express only few MHC II molecules and are weak T cell stimulators but highly endocytic. When BM‐DC cultures in GM‐CSF were treated with 1 μg / ml LPS at day 0 of the culture or throughout the culture, only immature DC developed as defined by phenotype (MHC II low) and function (high endocytosis, weak primary mixed lymphocyte reaction). Those early LPS‐treated immature DC induced alloantigen‐specific anergy of CD4⁺ T cells in vitro. These findings might contribute to the understanding of reduced T cell immunity in the course of septic shock and find application in DC‐mediated tolerogenic immunotherapy strategies.