THE USE of thorium dioxide in man has been controversial since its introduction in 1930.1 In the form of a colloidal suspension, thorium dioxide is a radiopaque diagnostic agent of greatest usefulness in visualizing the liver and spleen. Numerous observers have shown that this particulate material is taken up by the reticuloendothelial system after intravenous injection.2 The bulk of it is thus concentrated in the liver and spleen, which are then readily visible roentgenographically. In many patients it has been possible to demonstrate such lesions as liver abscesses and tumors without recourse to surgical measures.3 In this technique of hepatosplenography thorium dioxide is a unique agent. Some observers have recognized potential dangers in the use of thorium dioxide, in view of its prolonged radioactivity and apparently permanent domicile in the body.4 In experimental animals it has been possible to produce malignant tumors with the use of