The ability of human liver to oxidize pristanic acid was investigated. Liver from control subjects was found to contain pristanic acid oxidase activity, an H2O2-producing enzyme activity not previously demonstrated in mammals. In livers from patients with the cerebro-hepato-renal syndrome of Zellweger, a genetic disease characterized by the absence of morphologically distinguishable peroxisomes, pristanic acid oxidase activity was found to be deficient. These results indicate that pristanic acid is oxidized in peroxisomes rather than in mitochondria as believed until now. Furthermore, our findings provide an explanation for the elevated levels of pristanic acid in body fluids from patients lacking peroxisomes.