The revolutionary effect on the diagnosis of gastrointestinal conditions which was made possible by the use of the opaque meal has given rise repeatedly to the idea that if, by some means, an opaque substance could be safely introduced into the gallbladder so that its contour could be seen with the roentgen ray, the diagnosis of many obscure and doubtful cases of cholecystitis might be made easy and accurate. To fulfil the necessary practical requirements, the opaque substance must be something that is excreted into the bile after being injected either subcutaneously or intravenously or after being given by mouth; and, furthermore, it must be devoid of toxic effects when used in the concentrations necessary for the shadow of the gallbladder to be seen. The extensive use of various dyes in the search for a test of liver function has revealed the fact that certain ones are excreted almost entirely