The better understanding of antitumour immunity has improved our knowledge concerning the mechanisms of action of BCG. There are three phases in the immune response to BCG: first of all, BCG adheres and is then phagocytosed by antigen-presenting cells, but also by urothelial cells. This phase corresponds to the early release of so-called inflammatory cytokines (IL-1, IL-6, IL-8). These cytokines may be responsible for certain adverse effects, but may also participate in the cytotoxic phenomenon. The second phase consists of recognition of bacterial antigens by CD4 lymphocytes, which release mainly IL-2 and IFN-gamma(TH1 response). This cell activation leads to the third phase, which consists of amplification of cytotoxic-populations: CD8, gamma delta lymphocytes, macrophages, NK, LAK, BAK. All these cells also release cytokines which then regulate the response. The identification of these modulations allows rationalization of BCG instillation protocols, but identification of the truly cytotoxic elements would allow the proposal of more effective immunization protocols.