A new angiographic contrast medium, sodium iothalamate 80%, has been tested in experimental animals by standardized toxicity tests. The results were compared to results obtained with other currently employed concentrated media. Sodium iothalamate proved least toxic of any medium tested. This new medium has high radiodensity and low viscosity. In 248 clinical angiograms sodium iothalamate proved superior for all uses involving intravascular injection of concentrated contrast media.