Ionizing radiation, which includes X-rays and gamma rays, is used in a variety of different applications, including in human health. However, high levels of exposure can result in significant morbidity and mortality in humans. Molecular and nanoscale systems that can detect different levels of ionizing radiation can ultimately lead to effective sensing platforms for a variety of applications. In the current work, we describe the use of polypeptide-templated formation of bimetallic metal nanoparticles with potential applications as a colorimetric reporter of elevated levels of ionizing radiation typically used in blood irradiation. Cysteine-containing elastin-like polypeptides were employed together with reducing agents in order to engender radiation-facilitated formation of bimetallic gold–silver bimetallic nanoparticles from a mixture of their respective metal salts. This formation of colored nanoparticle dispersions from colorless metal salt solutions acted as a visual reporter of ionizing radiation in the dose range of 25–100 Gy. Nanoparticles were characterized using UV–visible spectroscopy, transmission electron microscopy, elemental analyses and dynamic light scattering. Our results indicate that polypeptide-bimetallic nanoparticle systems may be attractive reporters of elevated levels of ionizing radiation.