Cobrotoxin, the crystalline toxic protein obtained from Formosan cobra venom, is immunochemically homogeneous and was proved to be the main toxic protein in the venom. One hundred per cent precipitable antibody was isolated immunospecifically by gel filtration on Sephadex G-100 after the antigen-antibody complexes had been dissociated with 0.53 M formic acid-0.15 M NaCl, pH 2.05. The specific neutralizing capacity of the purified antibody was 17.5 greater than that of the antisera. The purified antibody was digested with papain and the univalent fragments formed soluble antigen-antibody complex with the homologous antigen. An estimate of the molecular weight of the soluble complex formed from cobrotoxin and Fragment I indicated that cobrotoxin consists of three antibody-combining sites/molecule.