The presence of immunoreactive xanthine oxidase protein was proven in a xanthinuric patient, using a polyclonal antibody against xanthine oxidase. The antibody was raised against purified human liver xanthine oxidase in a rabbit. Double immunodiffusion method demonstrated the existence of an immunologically reactive xanthine oxidase which did not possess xanthine oxidase activity. In addition, urinary excretion of oxypurines in the patient and her family was investigated. The results indicated that a brother and a sister had xanthinuria.