The stem latex of a medicinally important plant, Euphorbia nivulia was successfully used to induce room temperature/microwave synthesis of silver and copper nanoparticles even at high concentrations. The major component of the latex, Euphol, is assumed to be the reducing moiety; while stabilization is assisted by certain peptides and terpenoids present within the latex as supported by the FT-IR analysis. The fast and simple process has high reproducibility and leads to formation of nanoparticles with 5–10nm diameter. The one step synthesis can be extended for other metals. The nanoparticle solutions being completely free of toxic chemicals can be directly used for antimicrobial tests. The as synthesized solutions of both metals exhibited excellent bactericidal action against both gram negative and gram positive bacteria well below the in vitro cytotoxic concentration. The non-cytotoxic metal–latex aqueous solution offers a rational approach towards antimicrobial application and for integration to biomedical devices.