Ornithine decarboxylase [EC. 4.1.1.17] was purified 3.5 X 10(5)-fold with an overall yield of 8% from livers of rats treated with thioacetamide and DL-alpha-hydrazino-delta-aminovaleric acid. The enzyme preparation was homogeneous as judged by three criteria, namely polyacrylamide gel electrophoresis with or without sodium dodecyl sulphate and Ouchterlony double immunodiffusion analysis. The final specific activity was 1.1 X 10(6) nmol/h mg protein, which closely coincided with the value predicted by Pritchard et al. (16). The enzyme was composed of two identical subunits and the molecular weight was estimated to be 105,000 by polyacrylamide gel electrophoreses at several different gel concentrations. It was shown by an immunochemical method that diet-induced change in hepatic ornithine decarboxylase activity was due mostly, if not totally, to the change in the amount of enzyme protein.