Nanomedicine concerns the use of precision-engineered nanomaterials to develop novel therapeutic and diagnostic modalities for human use. The present study demonstrates the efficacy of silver nanoparticles (AgNPs) biosynthesis from leaf extract of Vitex negundo L. as an antitumor agent using human colon cancer cell line HCT15. The AgNPs synthesis was determined by UV-visible spectrum and it was further characterized by field emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDX), transmission electron. microscopy (TEM), X-ray diffraction (XRD) and fourier transform infrared spectroscopy (FTIR) analysis. The toxicity was evaluated using changes in cell morphology, cell viability, nuclear fragmentation, cell cycle and comet assay. The percentage of cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Our results showed that biosynthesized silver nanoparticles inhibited proliferation of human colon cancer cell line HCT15 with an IC50 of 20 mu g/ml at 48 h incubation. AgNPs were shown to promote apoptosis as seen in the nuclear morphological examination study using propidium iodide staining and DNA fragmentation by single cell gel electrophoresis technique. Biosynthesized AgNPs arrested HCT15 cells at G(0)/G(1) and G(2)/M phases with corresponding decrease in S-phase. These results suggest that AgNPs may exert its antiproliferative effects on colon cancer cell line by suppressing its growth, arresting the G(0)/G(1)-phase, reducing DNA synthesis and inducing apoptosis. (C) 2013 Elsevier Ltd. All rights reserved.