A novel site targeted acoustic contrast agent with the potential to define and localize a myriad of pathological tissues for diagnosis or therapy has been developed. The agent is a lipid-coated perfluorocarbon microemulsion modified to accommodate avidin--biotin conjugation. The formulation has low inherent echogenicity in suspension, yet high acoustic reflectivity after binding to a surface or itself. Targeted and control emulsions were suspended in buffered saline and imaged with a 7.5-MHz linear array transducer. Neither suspension manifested significant ultrasonic backscatter until avidin was added. Avidin induced aggregation and produced a marked enhancement in backscatter from the targeted but not from the control emulsion. Targeted and control emulsions were exposed to avidin conjugated nitrocellulose membranes and imaged with a high-resolution 50-MHz acoustic microscope. Bound targeted emulsion produced a fourfold enhancement in the magnitude of ultrasonic backscatter (+6.3 dB) from the membrane relative to the control (p<0.05). Moreover, acoustic contrast targeted to thrombi both in vitro and in vivo using antifibrin monoclonal antibodies markedly enhanced (p<0.05) their acoustic reflectivity. These data demonstrate the first site targeted acoustic contrast agent and suggest potential improvement in the ultrasonic diagnosis of thrombi and other pathology.