1. Fasting levels of plasma cysteine, plasma sulphate and the plasma cysteine/sulphate ratio were measured in patients with primary biliary cirrhosis and compared with those in patients with other liver disease, general intensive therapy unit patients and healthy subjects. 2. Plasma cysteine was significantly elevated in patients with primary biliary cirrhosis (median 0.364 nmol/mg of protein, P < 0.0001) and patients with other liver disease (median 0.445 nmol/mg of protein, P < 0.0001), compared with healthy control subjects (median 0.125 nmol/mg of protein) and increased progressively with the severity of liver disease. Plasma cysteine was also elevated in intensive therapy unit patients (median 1.564 nmol/mg of protein) compared with healthy control subjects ( P < 0.0001) and patients with other liver disease ( P < 0.0001). 3. Plasma sulphate was reduced significantly only in patients with primary biliary cirrhosis (median 0.822 nmol/mg of protein) compared with healthy control subjects (median 1.37 nmol/mg of protein, P < 0.05). There was no significant difference in plasma sulphate between disease groups. 4. The plasma cysteine/sulphate ratio was significantly elevated in patients with primary biliary cirrhosis (median 0.448, P < 0.0001) and patients with other liver diseases (median 0.394, P < 0.0001) compared with healthy control subjects (median 0.095). The ratio was also elevated in intensive therapy unit patients (median 1.650, P < 0.0001) compared with healthy control subjects and liver disease groups ( P < 0.0001). 5. In conclusion, plasma cysteine rises in primary biliary cirrhosis and other forms of liver disease. This effect is not specific to liver disease, since cysteine is elevated in an heterogeneous group receiving intensive care. Impairment of trans -methylation and trans -sulphuration pathways does not explain the finding of increased plasma cysteine. Since cysteine is elevated in non-hepatic disease, it may reflect the effect of muscle breakdown and the catabolic state. Impaired activity of cysteine dioxygenase and impaired mitochondrial function may be contributory, but this requires further study. These metabolic changes may reflect progressively diminished detoxification capacity within the liver and other tissues.