The molecular weight and subunit structure of an ultracentrifugally and electrophoretically pure preparation of human ceruloplasmin were studied. A molecular weight of 124,000 was determined for the native protein by sedimentation equilibrium. It was found that the protein molecule dissociates into three smaller species L, H, and H′ by (a) sodium dodecyl sulfate in the presence of 2-mercaptoethanol or p-hydroxymercuribenzoate or (b) urea or Gdn · HCl in the presence of 2-mercaptoethanol. The molecular weights of the L, H, and H′ species, determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, are 16,000, 53,000, and 69,000 respectively. Removal by dialysis of the dissociating agent sodium dodecyl sulfate and 2-mercaptoethanol brings about reassociation, and recombination with copper leads to reconstitution of ceruloplasmin as evidenced by the restoration—up to 80%—of the original oxidase activity. The dissociation and reconstitution experiments unequivocally show that human ceruloplasmin is a multichain protein. It is proposed that ceruloplasmin is an L2H2 tetramer, where L and H are light and heavy chains of molecular weights 16,000 and 53,000, respectively.