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An enzyme preparation obtained from brain tissue catalyzes the stereospecific reduction of crotonyl CoA to butyrate. Tritium from TPNH3 was localized predominantly on the β-carbon of recovered butyric acid. Both this enzyme and a preparation from adipose tissue, which also catalyzes the reduction of crotonyl CoA, catalyze the synthesis of long-chain fatty acids from malonyl CoA and acetyl CoA. The significance of these findings is considered in view of the demonstration that, in long-chain fatty acids synthesized in the presence of TPNH3, labeled hydrogen is localized on alternate carbon atoms beginning with the β-carbon.