Intravesical bacillus Calmette-Guerin (BCG) therapy is considered the most successful immunotherapy against solid tumors of human bladder carcinoma. To determine the actual effector cells activated by intravesical BCG therapy to inhibit the growth of bladder carcinoma, T24 human bladder tumor cells, expressing very low levels of class I MHC, were co-cultured with allogeneic peripheral blood mononuclear cells (PBMCs) with live BCG. The proliferation of T24 cells was markedly inhibited when BCG-infected dendritic cells (DCs) were added to the culture although the addition of either BCG or uninfected DCs alone did not result in any inhibition. The inhibitory effect was much stronger when the DCs were infected with live BCG rather than with heat-inactivated BCG. The live BCG-infected DCs secreted TNF-alpha and IL-12 within a day and this secretion continued for at least a week, while the heat-inactivated BCG-infected DCs secreted no IL-12 and little TNF-alpha. Such secretion of cytokines may activate innate alert cells, and indeed NKT cells expressing IL-12 receptors apparently proliferated and were activated to produce cytocidal perforin among the PBMCs when live BCG-infected DCs were externally added. Moreover, depletion of gammadelta T-cells from PBMCs significantly reduced the cytotoxic effect on T24 cells, while depletion of CD8beta cells did not affect T24 cell growth. Furthermore, the innate effectors seem to recognize MICA/MICB molecules on T24 via NKG2D receptors. These findings suggest the involvement of innate alert cells activated by the live BCG-infected DCs to inhibit the growth of bladder carcinoma and provide a possible mechanism of intravesical BCG therapy.