Albunex® is an ultrasound contrast agent prepared by sonicating 5% human serum albumin to produce stable, air‐filled, albumin microspheres. It is an effective ultrasound contrast agent for use with echocardiography and other ultrasound radiology procedures. The characteristics of Albunex that are responsible for the clinically observed high echogenicity of Albunex are discussed. Albunex is composed of a high concentration of noncoalescing microspheres (400–500 μ/nl) with a mean diameter of ≈4 μm and a shell thickness of 30–50 nm, as determined by particle analysis, light microscopy, and electron microscopy. Attenuation and backscatter measurements as a function of ultrasonic frequency and amplitude are reported for varying concentration and size distributions of Albunex.