Interaction of single-cell protein of Spirulina platensis with aqueous AgNO3 and HAuCl4 was investigated for the synthesis of Ag, Au and Au core-Ag shell nanoparticles. Biological reduction and extracellular synthesis of nanoparticles were achieved in 120 h at 37 degrees C at pH 5.6. The nanometallic dispersions were characterized by surface plasmon absorbance measuring at 424 and 530 nm for Ag and Au nanoparticles, respectively. For bimetallic nanoparticles, absorption peak was observed at 509, 486 and 464 nm at 75:25, 50:50 and 25:75 (Au:Ag) mol concentrations, respectively. High-resolution transmission electron microscopy showed formation of nanoparticles in the range of 7-16 (silver), 6-10 (gold) and 17-25 nm (bimetallic 50:50 ratio). XRD analysis of the silver and gold nanoparticles confirmed the formation of metallic silver and gold. Fourier transform infrared spectroscopic measurements revealed the fact that the protein is the possible biomolecule responsible for the reduction and capping of the biosynthesized nanoparticles.