Z protein was purified from the rabbit and rat liver by fractionation with ammonium sulfate. The physico-chemical properties of Z protein was studied by SDS (sodium dodecyl sulfate) polyacrylamide gel electrophoresis, polyacrylamide gel electrofocusing, amino acid analysis, and sedimentation analysis. The molecular weight was determined to be 1.2×10⁴ by gel filtration on Sephadex G-75 and 1.1×10⁴ by SDS-polyacrylamide gel electrophoresis, and also 8872 and 8639 from amino acid analysis with the rabbit and rat liver, respectively. Z protein lacked tyrosine and tryptophan residues, and had an absorption maximum at 260-270 nm. The isoelectric point was at pH7.0. The sedimentation coefficient of Z protein was ca. 0.7S at a concentration of 0.04-0.06%. Physical and chemical properties of rabbit and ratliver Z protein were closely alike.