The intracellular localization studies of human blood platelets by sucrose gradient and differential centrifugation, showed that palmitoyl-CoA hydrolase was mainly localized in the cytosol fraction. However, a localization also in the mitochondrial fraction seems possible as disruption of platelets by the French press and nitrogen decompression techniques resulted in mitochondrial damage. In human liver the palmitoyl-CoA hydrolase was localized in the mitochondrial and microsomal fractions. Palmitoyl-CoA synthetase was localized in the mitochondrial and microsomal fractions in both platelets and liver. The possible physiological implications of these differences, and the finding of a very high ratio of palmitoyl-CoA hydrolase/palmitoyl-CoA synthetase in platelets compared with liver, are discussed.