Dendritic cells (DC) play a central role in the development of an adaptive immune response against tumor. In addition to its role in antigen presentation, DC also possesses cytotoxic activity against tumor cells. We have earlier shown phenotypic and functional maturation of bone marrow derived dendritic cells (BMDC) by G1-4A, an arabinogalactan derived from Tinospora cordifolia. In this study, we have investigated the killer phenotype of BMDC matured in the presence of G1-4A, [mBMDC (G1-4A)] on tumor cells. We have observed several fold increase in killing of tumor cells by mBMDC (G1-4A). The tumoricidal activity was not specific to syngeneic tumors cells but could kill xenogenic tumors also. Nitric oxide released by mBMDC (G1-4A) generates peroxynitrite in tumor cells and is responsible for killing of target cells. This killing was completely abrogated by inducible nitric oxide synthase (iNOS) inhibitor 1400W and NADPH oxidase inhibitor apocyanin. The killed target cells are phagocytosed by BMDC which further activate syngeneic cytotoxic T cells. These results thus show that G1-4A treated mBMDC acquire killer phenotype along with maturation which plays an important role in activation of cytotoxic T cells.