Recent reviews have dealt in detail with therapeutic embolization [1 -3]. This technique offers a wide range of therapeutic possibilities especially for cases of hemorrhage, tumor, and vascular malformation; new applications are continually being introduced [3-6]. Remarkable technical progress in recent years has resuIted from continuously improved catheters and better tolerated radiopaque products. In addition, microencapsulation techniques have been adapted lately for preparation of better defined microemboli [6, 7]. Thus, a new dimension was introduced, permitting embolization of very fine vessels, such as those occurring with intratumoral vasculature. On the basis of experience accumulated over the years and from sideeffects encountered in clinical practice, it is possible to list the properties required of an outstanding embolic material. They are: chemical and toxicologic safety; chemical, biologic, and mechanical stability; sterility; thrombogenicity; radiopacity; adherence to vascular intima; nonadherence to walls of catheter; availability in various well defined sizes; and dispersibility in blood or aqueous media. The technique we describe permits the preparation of microemboli fulfilling most of these requirements.