1. The strength of binding of 64Cu to the following was determined by dialysis against solutions of buffered sodium chloride with and without EDTA, penicillamine and sodium glycocholate. (a) Human albumin, serum and bile labelled in vitro from patients with and without Wilson's disease, (b) Rat bile labelled in vivo and in vitro. 2. Serum and albumin were found to bind copper comparatively weakly. 3. Bile from all sources contains a non-diffusible fraction with a very strong binding affinity for copper. 4. Bile from patients with Wilson's disease binds copper as strongly as normal bile.