Ultrasound is a noninvasive tool with which the physician can gain insight into the state of the internal organs of the body. A contrast agent is a substance which, when injected into the body improves the resolution of an image. The use of diagnostic ultrasound is limited without development of such an agent. This paper describes the development of a new class of contrast agent. The agent consists of microbubbles generated in a solution by sonication, and stabilized by a layer of nonionic surfactant molecules. A single surfactant type alone does not produce stable bubbles, and only certain combinations of surfactants, one with a hydrophile-lipophile balance (HLB) greater than 10.5 and the other with an HLB less than 9, are successful. An agent prepared from Span60 (R) and Tween80 (R) is described in detail. A Coulter analysis revealed that 90% of the bubbles were less than 10 mu m in diameter. This is essential if the agent is to pass the pulmonary capillary bed. B-mode imaging of a sample of microbubbles indicated that these bubbles were highly echogenic (that is they produced a strong contrast). A Langmuir trough study of the molecular arrangement of the surfactant molecules inside the microbubble skin suggested that there are 1.7 molecules of Span to each Tween molecule. Detailed analysis of the pressure-area curves and area per molecule data lead to the hypothesis that the correct proportion of Span molecules can substantially reduce the head group repulsion found in the all-Tween situation, and this results in the generation of stable microbubbles. A molecular arrangement of these surfactant molecules around the microbubble is proposed.