Aliphatic mercaptans (aethanthiol, methanthiol, dimethylsulphide) can be measured in serum with a simple and rapid gaschromatographic method. The test takes 30 minutes. Aethantiol was found to be increased ten-fold (P less than 0.0001) in patients with acute hepatic failure (endogenous coma), while in exogenous hepatic coma it was always normal or decreased. Mild increase in aethanthiol concentration (two or threefold) was also found in chronic aggressive hepatitis, cirrhosis and obstructive jaundice. Methanthiol concentration was elevated in patients with endogenous and exogenous hepatic coma. Values for methanthiol are, however, of only limited use, because methionine is converted in small amounts to methanthiol during the test procedures. Dimethylsulphide is found in only very severe cases of endogenous or exogenous hepatic coma and can be considered to be a prognostically unfavourable sign. Determination of mercaptans makes it possible to differentiate exactly between endogenous and exogenous hepatic coma. Its value also lies in the recognition of the severity of endogenous intoxication and it is suitable for serial and control determination of the effectiveness of therapeutic measures.