The peroxisomes in the liver of four patients with alcoholic hepatitis and in six patients with drug-induced hepatitis are compared to eight control livers by catalase cytochemistry and morphometry. A decrease of catalase activity is observed in alcoholic, amitriptyline, aprindine, clomipramine and methiomazole hepatitis. Peroxisomes with a heterogeneous distribution of the catalase reaction product are found in most hepatitis livers. The number of organelles is increased 1.5 to 4.2 times in alcoholic, aprindine, methimazole and phenytoin hepatitis livers. In the last case, peroxisomes are also smaller. Changes in shape are seen in all hepatitis livers; they include invaginations, tails, funnel-like constrictions and gastruloid cisternae. In aprindine, phenytoin, methimazole and two alcoholic hepatitis livers, surface density exceeds the upper control value. These data indicate a loss of catalase activity in most hepatitis livers but also peroxisomal proliferation and shape modifications. It has been proposed that the latter changes are favorable for metabolic activity.