Nearly monodispersed spherical particles of silica were synthesized and coated with thin layer of silver nanoparticles. Silver coated silica particles, forming core-shell particles exhibited a strong surface plasmon resonance peak at 453 nm. A very small amount (20 mug) of rabbit immunoglobulin in core-shell particle solution results in to a marked shift in surface plasmon resonance. Addition of 20 mug quantity of goat anti rabbit antibodies results in to a red shift of surface plasmon resonance to 494 nm. This demonstrates that silver coated silica particles are sensitive probes for rapid antibody-anti antibody kind of interaction investigations. Fourier transform infra red spectroscopy and scanning electron microscopy have been used to interpret the optical extinction spectroscopy results. (C) 2005 Elsevier B.V. All rights reserved.