Abstract A method for the detection, localization, and quantification of left-to-right circulatory shunts by the inhalation of an inert radioactive gas, Kr 85, is described. In the course of cardiac catheterization patients inhaled the gas for thirty seconds as integrated blood samples were drawn simultaneously from a systemic artery and a right heart chamber or the pulmonary artery. Fifty-six Kr 85 tests were performed in forty-five patients without left-to-right shunts. Among these control tests the radioactivity of the pulmonary arterial or right heart sample was always less than 15 per cent of that in the arterial sample. Of the sixty-nine tests performed in patients with proved shunts the ratio exceeded 20 per cent in all but six instances, when it was between 15 and 20 per cent. The magnitude of the shunt could also be estimated by comparing pulmonary and systemic arteriovenous differences in Kr 85 content. In all patients N 2O tests were performed in the same chambers in which Kr 85 tests were performed, and in all patients with shunts determinations of blood oxygen content were also made. The results of the Kr 85 tests were comparable in every instance to those of the N 2O tests and both were superior in diagnostic accuracy to the oxygen method. With the Kr 85 test, shunts resulting in pulmonary to systemic flow ratios of 1.1:1 could uniformly be detected. The Kr 85 content of blood samples was determined with a Geiger counter and the results of the test were available within a few minutes. The Kr 85 test thus combines the accuracy of the N 2O test with a simple and rapid technic of analysis.