1. The method of ultrasonic imaging in which microbubbles are injected into a mammal to alter the accoustic properties of a predetermined area which is then ultrasonically scanned to obtain an image of the area for use in medical procedures, wherein the improvement comprises the steps of forming an aqueous protein solution, subjecting said solution to high frequency sonication while heating the solution sufficiently to denature portions of the protein, said sonication forming microbubbles of relatively uniform size stabilized by the denatured protein, and using the stabilized microbubbles as an injectate for said ultrasonic imaging.