A hemorrhagic principle (HR1B) isolated from the venom of Habu snake (Trimeresurus flavoviridis) was detoxified with formalin at pH 5, 7 and 9. Higher pH shortened the period needed for detoxification and lowered the final formalin concentration required for complete detoxification. The extent of polymerization and the immunogenicity of the toxoid formed were dependent upon pH during detoxification; the lower the pH, the higher the extent of polymerization and the immunogenicity of the toxoid. The higher immunogenicity possessed by the toxoid with higher extent of polymerization was confirmed by fractionation of the toxoid prepared at pH 7 on Sephadex G-200 and by comparing the immunogenicity of the toxoid fractions. The presence of lysine during detoxification brought about a partially polymerized toxoid having new immunogenic characteristics and a higher immunogenicity.