Targeted microbubbles for ultrasound imaging were examined with an avidin-modified gold surface of a surface plasmon resonance (SPR) biosensor. Contrast media of ultrasound (US) imaging were fabricated with dispersions of biotinylated gases (microbubbles) in an aqueous medium and the binding interaction of a targeted microbubble contrast agent was characterized by the refractive index change of the adsorbed layers in the SPR biosensor. The shells of the microbubbles were consisted of a gas-filled layer of lipids and polypeptides. Avidin-biotin interaction as a typical protein-ligand model was investigated on biotinylated polypeptide lipid microbubbles that were functionalized with biotin by directly grafting them to poly(succinimide) (PSI). The targeted microbubbles were below 7 mu m (95%) and their mean diameter was 2.5 mu m. The affinity constant (K-A) of the biotinylated polypeptide lipid-microbubbles from the SPR data (K-A = 9.46 g(-1) L) was about 10 times greater than that of polypeptide lipid microbubbles (K-A = 1.00 g(-1) L). The SPR biosensor may help in the prediction of the in vivo behavior of targeted microbubbles (ultrasound contrast agent) in a flow system and in the design of successful target preparation.