Indocyanine green (ICG) is a tricarboxyanine dye used in the indicator dilution technique to measure cardiac output. Animal studies and preliminary human observations suggest that ICG may have properties useful for assessing liver function and hepatic blood flow. This study aimed to determine whether ICG has properties suitable for assessing liver function and hepatic blood flow in humans. The physical properties of ICG were studied, including its volume of distribution and absorption spectra in different solutions. ICG was found to bind to plasma proteins, primarily albumin, and its absorption spectrum was similar in human plasma and albumin solution. Starch block electrophoresis showed that ICG migrated with albumin and bound to other plasma proteins in small amounts. ICG was not excreted in urine but was found in bile. It was cleared from plasma rapidly, with a decay rate of 18.5% per minute in normal subjects, compared to 13.8% per minute for sulfobromophthalein (BSP). In patients with cirrhosis, ICG decay was slower, indicating impaired liver function. ICG was not cleared by the kidneys and had negligible peripheral tissue uptake. Hepatic extraction of ICG was studied in seven subjects without liver disease. ICG was extracted efficiently by the liver, with extraction rates ranging from 0.59 to 0.83. Estimated hepatic blood flow ranged from 0.70 to 1.80 L per minute per square meter of body surface area. ICG appears to be rapidly and completely bound to plasma proteins, with no significant irritation upon subcutaneous introduction. In normal subjects and selected patients with liver disease, ICG decay was similar to that of BSP. In patients with cirrhosis, the correlation between initial decay rates of ICG and BSP was 0.92, indicating good discriminative value in selecting those with significant liver disease. ICG provides a reliable means of estimating hepatic blood flow using the Fick principle. Its physical properties, degree of hepatic removal, tolerance by human subjects, and rapid decay even in the presence of mild liver disease make it an ideal dye for this purpose. ICG is not cleared by extrahepatic mechanisms in detectable amounts and is not irritant. It is an excellent substance for estimating hepatic blood flow by the Fick principle.Indocyanine green (ICG) is a tricarboxyanine dye used in the indicator dilution technique to measure cardiac output. Animal studies and preliminary human observations suggest that ICG may have properties useful for assessing liver function and hepatic blood flow. This study aimed to determine whether ICG has properties suitable for assessing liver function and hepatic blood flow in humans. The physical properties of ICG were studied, including its volume of distribution and absorption spectra in different solutions. ICG was found to bind to plasma proteins, primarily albumin, and its absorption spectrum was similar in human plasma and albumin solution. Starch block electrophoresis showed that ICG migrated with albumin and bound to other plasma proteins in small amounts. ICG was not excreted in urine but was found in bile. It was cleared from plasma rapidly, with a decay rate of 18.5% per minute in normal subjects, compared to 13.8% per minute for sulfobromophthalein (BSP). In patients with cirrhosis, ICG decay was slower, indicating impaired liver function. ICG was not cleared by the kidneys and had negligible peripheral tissue uptake. Hepatic extraction of ICG was studied in seven subjects without liver disease. ICG was extracted efficiently by the liver, with extraction rates ranging from 0.59 to 0.83. Estimated hepatic blood flow ranged from 0.70 to 1.80 L per minute per square meter of body surface area. ICG appears to be rapidly and completely bound to plasma proteins, with no significant irritation upon subcutaneous introduction. In normal subjects and selected patients with liver disease, ICG decay was similar to that of BSP. In patients with cirrhosis, the correlation between initial decay rates of ICG and BSP was 0.92, indicating good discriminative value in selecting those with significant liver disease. ICG provides a reliable means of estimating hepatic blood flow using the Fick principle. Its physical properties, degree of hepatic removal, tolerance by human subjects, and rapid decay even in the presence of mild liver disease make it an ideal dye for this purpose. ICG is not cleared by extrahepatic mechanisms in detectable amounts and is not irritant. It is an excellent substance for estimating hepatic blood flow by the Fick principle.