DC are unique antigen presenting cells, and their ability to induce proliferation of T cells in a mixed leukocyte reaction (MLR) assay is commonly used for the evaluation of their function. To determine the mechanisms involved in DC-induced T cell activation in a primary MLR assay, a variety of different agents were examined in this study that interfere with DNA synthesis, membrane organization, protein synthesis, and maturation induced by bacterial products. While only live DC were able to induce T cell proliferation in the MLR assay, irradiation of DC did not influence their stimulatory capacity. Fixation of DC membrane with paraformaldehyde resulted in a loss of DC capacity to induce T cell proliferation demonstrating that physical organization of the plasma membranes plays an important role in the induction of T cell activation. In addition, the pretreatment of DC with cycloheximide revealed that protein synthesis was not critical for the ability of DC to activate T cells. Finally, Staphylococcus aureus-mediated activation of DC significantly increased T cell proliferation and this effect was not dependent on IL-12 production of DC since DC generated from IL-12 knockout mice were not different from wild type DC. In summary, these data suggest that DC membrane structures are responsible for the antigen presentation and co-stimulation and play a key role in T cell recognition and activation by DC.