Different methods of performing the (/sup 14/C) aminopyrine breath test have been assessed. A tracer dose of 2 ..mu..Ci without a loading dose and with a single breath collection at two hours was the method selected, since it gave the best discrimination between patients with hepatocellular diseases and normal subjects (5.2 +- 0.2%, mean - SEM). Reduced values occurred in patients with chronic active hepatitis (with and without cirrhosis) (1.5 +- 0.2%), alcoholic cirrhosis (1.7 +- 0.4%) and hepatitis (2.5 +- 0.3%), and late primary biliary cirrhosis suggesting defective microsomal function with respect to demethylation. Normal results were common in early primary biliary cirrhosis. Two weeks of prednisolone therapy caused some improvement in the breath test in nine of ten patients with chronic active hepatitis. It is concluded that the (/sup 14/C) aminopyrine breath test is a simple test for detecting hepatocellular dysfunction, but has no obvious diagnostic advantage over the determination of serum aspartate transaminase and two hour post-prandial bile-acids.