Five patients showing a normal to near-normal BSP test and a marked delay in ICG plasma clearance, and two families with clustering of the same abnormality, are described. Two had Gilbert's syndrome, one was convalescing from acute hepatitis, and the other two had no detectable liver abnormality. Measured indices of ICG metabolism indicated a marked reduction in the hepatic uptake, storage capacity, biliary transport maximum, and an increased reflux into plasma. Biochemical studies on the binding of ICG by plasma proteins failed to demonstrate any difference from normal controls. Thus, the primary defect seems to be in the transport of ICG by the hepatocyte. Although the relationship of the defect in ICG metabolism to other constitutional hyperbilirubinaemias is not clear, it is possible that the defect in these patients is a constitutional one involving some steps in the hepatic disposal of organic anions.