We report on a radio opaque substance for the embolization of kidney tumours which consists of amino acids mixed up with a contrast medium. It is relatively easy to be injected and can be well directed because of its radio-opacity. Past and recent histological examinations of embolized pig kidneys show that this material provides a durable closure of the vessels and that there is no essential tendency of intravasal dissolution. It is therefore also suitable for palliative embolizations. The first results in clinical trial have proved to be encouraging.