1. A diketo acid hydrolase was purified about 100-fold from rat liver and some of its properties were studied. 2. Comparison of its activity with 3,5-dioxohexanoate (triacetate) and fumarylacetoacetate at various stages during the purification suggests that it is identical with 4-fumarylacetoacetate fumarylhydrolase. 3. The acetoacetate formed on cleavage of 3,5-dioxohexanoate by the hydrolase can be determined either with 3-hydroxybutyrate dehydrogenase or with 3-oxo acid CoA-transferase and 3-hydroxyacyl-CoA dehydrogenase. These reactions form the basis of enzymic assays for 3,5-dioxohexanoate.
