Xanthine dehydrogenase (EC 1.1.1.204), the rate-limiting enzyme of purine degradation, was purified 642-fold to homogeneity from liver of male Wistar rats. Antibody was generated to the purified enzyme in white rabbits and was partially purified. For the immunotitration a radioassay of high sensitivity was developed to determine low enzyme activities. Titration curves with the antibody showed that the xanthine dehydrogenase enzyme protein amounts in slowly growing hepatoma 20 and rapidly growing hepatoma 3924A were 34 and 4% of those of normal liver, which was in good agreement with the decrease in the activity of the enzyme to 33 and 2%, respectively. The contents of flavin adenine dinucleotide, the essential cofactor of the enzyme, in the immunoprecipitates in hepatomas 20 and 3924A were 27 and 4% of that of the normal liver. This is the first report to provide immunological evidence that a decreased enzyme activity in rat hepatomas, that of xanthine dehydrogenase, was due to a decrease in the enzyme protein amount. The markedly decreased xanthine dehydrogenase activity and amount have far-reaching biochemical and pharmacological implications for the tumors.