An impaired sulfoxidation pathway has been implicated in the pathogenesis of chlorpromazine‐induced hepatotoxicity. Since some patients with chronic chlorpromazine‐induced cholestasis may have features of primary biliary cirrhosis, we studied the ability to sulfoxidate the amino acid analogue S‐carboxymethyl‐cysteine in 44 patients with primary biliary cirrhosis and in two control groups—one without liver disease and one with a variety of liver diseases other than primary biliary cirrhosis. Poor sulfoxidation was observed in 84 percent of the patients with primary biliary cirrhosis, as compared with 24 percent of patients with other liver diseases and 22 percent of normal controls (P < 0.0005 for both comparisons). Poor sulfoxidation did not correlate with the degree of hyperbilirubinemia or histologic severity of liver disease in any of the groups studied. There was an inverse correlation with age only in the patients with primary biliary cirrhosis (r = −0.44, P < 0.001). Liver transplantation was performed in six of the patients and improved sulfoxidation in five; in the four with primary biliary cirrhosis, sulfoxidation improved from poor to good or intermediate.