In a group of 16 patients, receiving chenic acid (20 mg/kg/day) and phenobarbital (60 mg/day) for gallstone dissolution, three patients were observed to develop a marked increase in serum levels of SGOT-SGPT and alkaline phosphatase without associated symptoms. Liver biopsy in one patient showed cellular infiltration with neutrophils and eosinophils, suggestive of a hypersensitivity reaction. Enzyme levels returned to normal, following cessation of chenotherapy, and a second biopsy 3 months later was normal. In that patient a challenge with chenic acid, at 250 mg/day, was again followed by a marked elevation in levels of SGOT-SGPT and alkaline phosphatase. These patients are the first to show marked elevation in liver enzyme levels during chenotherapy; the mechanism and significance are unclear.