Recently nanotechnology has generated a great deal of interest from scientists and engineers of nearly all disciplines. A simple green protocol for synthesis and immobilization of noble metal nanoparticles (gold and silver) from the leaf extract of Cassia auriculata and its impact on microbes, human cancer cell lines and plants have been described. The phytochemical analysis of leaf extract revealed phenols, flavonoids, tannins, cardiac glycosides and saponins. Two low molecular weight proteins of 15kD and 42kD bound with gold and silver nanocomposites were determined. Preparation of bionanocomposites includes immobilization of gold and silver nanocomposites on biopolymer film with leaf extract as reducing agent.