Abstract: An ultrasonic imaging agent is formed by subjecting an aqueous protein solution to high frequency sonication while heating the solution sufficiently to denature portions of the protein, thereby forming a dispersion of microbubbles stabilized by denatured protein. In a preferred embodiment, the protein solution is formed from human serum albumin, and the formed microbubbles have a relatively uniform size in the 2 to 5 micron range, suitable for imaging of pulmonary or small organ vasculature.