Chicken antithrombin was purified from fresh chicken plasma by affinity chromatography using heparin-agarose, and its amino acid and carbohydrate compositions, amino-terminal sequence, inhibition of human thrombin, and immunological properties were studied and compared with previously studied mammalian antithrombins (human, pig, rabbit, and rat), and also with chick ovalbumin. Chicken antithrombin is a single-chain glycoprotein with a total carbohydrate content of 17.5%, including 6.0% N-acetylglucosamine, 8.7% hexose, and 2.8% N-acetylneuraminic acid. The molecular weight estimated from sodium dodecyl sulfate(SDS)-polyacrylamide gel electrophoresis was 60,000. The amino-terminal sequence has been determined as Ala-Pro-Tyr-Ala-Val-Glu-Asp-Ile-Cys-Thr-Ala-Lys-Pro-Thr-Asp-Ile-Pro-Val-Asn, which is highly homologous to the terminal sequences of mammalian antithrombins, although the first 4 residues are quite different from those of mammalian species. Chicken antithrombin showed a stoichiometric inhibition against thrombin. The apparent dissociation constant (K1) for the complex was 6.4 X 10(-8) M. No immunological cross-reactivity was observed between chicken and mammalian antithrombins. Ovalbumin, which Hunt and Dayhoff (Biochem. Biophys. Res. Commun. 95, 864-871, 1980) proposed should be grouped in the same superfamily as antithrombin, showed neither immunological cross-reactivity with antithrombin or with its carboxymethylated derivative, nor any effect on the thrombin-antithrombin interaction. Ovalbumin showed no inhibitory effect on porcine elastase, either.