The form in which copper is bound in bile was studied in bile from patients with Wilson’s disease, control subjects and rats. Bile was labelled with 64Cu. Sephadex column chromatography using gels G-200-G-10 showed a major 64Cu and protein peak of about 5,000 daltons and a minor peak of about 8,000 daltons. A high-molecular-weight peak (above 800,000 daltons) which was present when the eluting solution consisted of water or saline was shown to be an artefact which could be abolished by the addition of bile salt to the eluting solution. Bile from control subjects, Wilson’s disease patients and from the rats behaved in the same manner binding copper to low-molecular-weight proteins. The relationship of these findings to the pathogenesis of copper retention in Wilson’s disease is disc