Abstract Radioactive nickel chloride ( 63NiCl 2) was injected iv in rabbits (4.5 μg Ni/kg/day for 14–21 days), and serums which were obtained at 24 hr after the last injection were fractionated by column chromatography. Chromatography on Sephadex G-200 separated serum 63Ni into three distinct components: (1) macroglobulin-bound- 63Ni, ⋍1.2% of total serum 63Ni; (2) albumin-bound- 63Ni, ⋍92.2% of total serum 63Ni; and (3) ultrafiltrable- 63Ni, ⋍6.5% of total serum 63Ni. The fractions containing macroglobulin-bound- 63Ni were pooled, concentrated by ultrafiltration, and taken for chromatography on DEAE-cellulose. This procedure resulted in the isolation of a single 63Ni-labeled protein, nickeloplasmin. Purified nickeloplasmin migrated as a single protein band on acrylamide gel electrophoresis, and yielded a single precipition arc on immunoelec-trophoresis, using polyvalent antiserum against rabbit serum proteins. Immunoelectrophoresis using specific antiserums showed that rabbit serum nickeloplasmin is an α 1-macroglobulin.