Specific antiserum to a purified preparation of ornithine decarboxylase was prepared and successfully labeled with 125I. Monospecificity of this antiserum was attained by repeated precipitation with normal rat liver. The conditions for optimal antigen/antibody reaction were investigated. When the antibody was tested against either crude or purified regenerating rat liver enzyme a single precipitin line was observed both with Ouchterlony double diffusion plates and with immunoelectrophoresis. The specificity of the purified antiserum was also evaluated by sodium dodecyl sulfate gel electrophoresis of the 14C-labeled enzyme.antibody complex, the radioactivity of which appeared as a sharp peak in the 90,000 molecular weight region. Finally, an approximate antigen/antibody molar ratio was determined using labeled enzyme and labeled antiserum.