The activity of the enzyme threonine dehydrogenase, which catalyzes the conversion of l-threonine to aminoacetone, has been found to be decreased in the livers of tumor-bearing hosts. The activity of this enzyme decreased to barely detectable levels in the livers of animals bearing advanced Sarcoma 37. With progressive growth of Sarcoma 37 hepatic catalase and ALA dehydrase activity decreased before threonine dehydrogenase activity but never to as great an extent as threonine dehydrogenase. Experiments on parabiotic animals showed a decrease in hepatic threonine dehydrogenase activity in animals bearing adrenocortical Carcinoma 494, but no decrease in their normal parabiotic partners. This decrease in hepatic threonine dehydrogenase activity was seen in adrenocortical Carcinoma 494 when the tumor was only about 6–9 per cent of body weight, but occurred with Sarcoma 37 only after the tumor exceeded one–third of the carcass weight.