This communication explains the biosynthesis of stable silver nanoparticles (AgNPs) from Melia azedarach and its cytotoxicity against in vitro HeLa cells and in vivo Dalton's ascites lymphoma (DAL) mice model. The AgNPs synthesis was determined by UV-visible spectrum and it was further characterized by scanning electron microscopy (SEM), dynamic light scattering (DLS) and X-ray diffraction (XRD) analysis. Zeta potential analysis revealed stable AgNPs at -24.9 mV. UV visible spectrum indicated an absorption peak at 436 nm which reflects its specific Surface Plasmon Resonance (SPR). Biosynthesized AgNPs were predominantly cubical and spherical with an average particle size of 78 nm approximately as observed through SEM and DLS analysis, respectively. Cytotoxicity of biosynthesized AgNPs against in vitro Human epithelial carcinoma cell line (HeLa) showed a dose-response activity. Lethal dose (LD50) value was found to be 300 mu g/mL of AgNPs against HeLa cell line. Cytotoxicity against normal continuous cell line human breast lactating, donor 100 (H BL 100) was found only in increased concentration of both AgNPs and 5-FU. In addition, in vivo DAL mice model showed significant increase in life span, induction of apoptosis was evidenced by acridine orange and ethidium bromide (AO and EB) staining. (C) 2011 Elsevier Ltd. All rights reserved.