The methanol extract of grape seed nanoparticles is prepared from grape seeds washed in distilled water and oven-dried at 60° C. for 12 hours. The seeds are milled or ground to a powder and sieved to a maximum size of 0.355 mm. The powder is added to concentrated HCl and stirred at 3000 rpm at 30° C. for one hour, and then distilled water is added with stirring for an additional 2 hours. The mixture is filtered, and the marc is dried to recover grape seed nanoparticles. The nanoparticles are added to methanol at the rate of 100 mg/ml, left in a shaker for 24 hours at room temperature, centrifuged, filtered, and the resulting extract (the supernatant) is recovered. Agar well diffusion testing showed that the nanoparticle extract exhibited greater antibacterial activity than a methanol extract of grape seeds alone, and testing showed greater antioxidant levels in the nanoparticle extract as well.