Bacillus Calmette-Guerin (BCG) is considered to be a successful biotherapy for treating bladder cancer (BCa). However, the underlying mechanisms of BCG have not been completely clarified, to date. The role of macrophages in BCG therapy for BCa has still not been determinedin vivo. In the present study, the role and potential mechanism of BCG (0.25, 1.25 and 6.25 mu g/mouse; intravenous) immunotherapy for BCa was investigated in a NOD/scid(IL2Rg-/-)(NSI) mouse model by targeting macrophagesin vivo. Notably, it was observed that NSI mice with T24 BCa cells displayed high levels of the macrophage marker CD11b(+)F4/80(+)after injection via the tail vein of live BCG, as well as a significant reduction in tumor volume. The levels of the inflammatory and macrophage maturation cytokines, such as tumor necrosis factor-alpha, interleukin (IL)-1 beta, IL-6, IL-12P70, TNF superfamily member 11 and monocyte chemotactic protein 1, were significantly increased in the serum and the tumor supernatant compared to that in normal control subjects. Furthermore, BCG promoted the expression of the pro-differential genes Spi-1 proto-oncogene, early growth response protein 1, nuclear factor (NF)-kappa B and proto-oncogene c-Fos in bone marrow. In conclusion, these observations indicate that the injection of live BCG can target macrophages against bladder tumor growthin vivo. The mechanism is likely related to the promotion of macrophage maturation, immune activation and increased numbers of macrophages infiltrating the bladder tumor.