To date, the precise mechanism of intravesical immunomodulators remains unknown. In vitro, interferon alpha (IFN-alpha) acts directly on neoplastic cells and inhibits their proliferation while it induces their differentiation. Urothelium and transitional cell carcinoma (TCC) cells express IFN-alpha receptor, the density of which correlates with lesion grade. IFN-alpha reduces neo-microvascular density in the normal urothelium adjacent to the tumor after transurethial resection (TUR), possibly via inhibition of COX-1. Moreover, IFN-alpha induces the membrane expression of tumor-related antigens and MHC antigens, providing a basis for a cellular immune response. When given intravesically, IFN-alpha may result in local and systemic T cell and NK cell activation. By monitoring nitric oxide (NO) end-products in urine and evaluating inducible nitric oxide synthase (iNOS) expression immunohistochemically, we were able to show that IFN-alpha may induce urothelial iNOS expression with subsequent formation of peroxynitrite, which might contribute to the antineoplastic action of IFN-alpha. Bacillus Calmette-Guerin (BCG) is thought to bind to the bladder wall via interaction between the bacterial antigen 85 complex and fibronectin. Although systemic reactions (evolution of cellular immune response, systemic production of cytokines and oxygen free radicals) have been reported, a likely scenario is that exposure to BCG results in a massive local immune response, characterized by induced expression of cytokines in the urine and in the bladder wall, and by a marked infiltration of the bladder wall by granulocytes and mononuclear cells. BCG-induced changes in tumor cell phenotype render them able to act both as lymphokine-alphactivated killer cell targets and antigen presenting cells. Although BCG may act directly on the proliferation of tumor cells, helper and cytotoxic T cells and, most probably, NK cells are absolutely necessary for any antitumor effects. Tumor cell killing is mediated through FasLigand, perforin and TNF-alpha. In a recent study, we found that BCG up-regulated iNOS expression in normal human urothelium in vivo, suggesting a role for NO in BCG-mediated antitumor activity.