Liver function tests are indispensable for diagnosis of liver diseases. It is desirable to conduct several kinds of test for over-all evaluation of liver function, considering the various aspects of metabolism performed in the liver. Diagnosis of the existence and severity of liver diseases, especially in cases without jaundice and judgment of prognosis and effects of treatment must be made by various liver fuction tests. So far, the BSP loading test has been widely used as the most sensitive index of liver function, since BSP greatly contributes to the diagnosis of liver disease without jaundice and evaluation of prognosis. However, the clinical use of the BSP test seems to be decreasing, because side effects, such as pain and anaphylaxis-like shock, occasionally causing death, have been reported(1). Indocyanine-Green (ICG) appears to be replacing the BSP test as a safer dye for the dye excretion test(2,3), but it has not yet been widely used for clinical purpose because of problems of product stability and measurement procedures. The authors previously used 35S-BSP to analyze the mechanism of liver uptake of the dye and determined maximum hepatic uptake capacity of BSP in normal controls and for various liver diseases(4). At the same time it was noted that, in the presence of function disturbance, the liver cannot adequately handle even a tracer dose of BSP, and blood clearance of a tracer dose of BSP is not exclusively dependent on hepatic blood flow, but is also dependent on the hepatocellular function of dye uptake(5). BSP and ICG have been also used as a bilirubin analogue for analysis of clearance after a single intravenous injection(6) and to biliary excretion of the dye under continuous infusion in normal subjects as well as in various liver diseases(7). A clinical test must be safe and simple for routine use. The measurement of 35S-BSP is rather complicated while 131I-BSP, the basic properties of which were intensively studied(8), and which was used clinically as an agent for sequential liver scanning(9), is simple to measure. Therefore 131I-BSP is an ideal agent for a simple liver function test based on a single injection and a single blood sampling. In this paper it is confirmed that BSP tests have hardly been used for liver function studies at Tokyo University Hospital. As a simple and safe liver function examination which might replace the conventional BSP test, a 30-minute retention test with 131I-BSP was introduced and compared with the conventional BSP 45-minute retention test.