1. A method is described for the histochemical localization of leucine aminopeptidase in fresh frozen and paraffin embedded tissues. 2. Evidence is offered which supports the idea that the substrate l-leucyl-β-naphthylamide is hydrolyzed by leucine aminopeptidase. 3. A comparison is made between the various diazonium salts used to demonstrate the enzyme in tissue sections. The shortcomings of each coupling agent are given. While the one recommended in this report has many favorable properties, its slow rate of coupling constitutes a serious drawback in localization at the cellular level. Experiments are in progress to overcome this deficiency in the method. 4. The value and future potential of chelations with metal ions in histochemical methodology is discussed. 5. Leucine aminopeptidase was found to be present histochemically in many organs of man, guinea pig, dog, and rat.