Abstract: Litchi is a famous fruit indigenous to southern China. When the fruit is fully ripe it is beautifully bright red, but the color fades rapidly and turns dull brown a few days after being kept at room temperature. In order to find out what is the active agent responsible for the rapid browning of the fruit, a study was made of the oxidizing enzymes present in the fruit shell. Crude enzyme preparation was made by grinding fruit shells with water or with phosphate-citric acid buffer (pH 6.8) and then filtering. Enzyme activity was measured by manometric method with the Warburg apparatus. An enzyme was found which can oxidize catechol and hydroquinone at pH 6.8, but it cannot oxidize monophenols such as p-cresol and tyrosine. The enzyme in litchi pericarp was thus found to resemble laccase with respect to substrate specificity. Enzyme activity was much reduced at pH values below 6.8, being about one-half at pH 6.2 and about one-fourth at pH 5.0 as that at pH 6.8. Boiled fruit shell extract still had some oxidizing activity toward hydroquine, presumably due to the presence of some thermostable factors capable of oxidizing polyphenols in the crude extract. Copper chelating agents such as thiourea and sodium diethyldithiocarbamate (dieca) had peculiar effects on this enzyme. The enzyme from fruit shells of the variety "Bailazi" (白蜡子)was inhibited by thiourea in only a short period. The inhibition gradually diminished and totally disappeared within 20–80 minutes. The degree and duration of inhibition varied with the concentration of thiourea, the higher the concentration, the greater the degree and the shorter the time of inhibition would be. After the inhibition has disappeared, absorption of oxygen continued to increase to an amount higher than that of the control, resulting in a stimulation effect. No inhibition was shown by thiourea to the enzyme from fruit shells Of the variety “Huai-zhi” (淮枝), which was stimulated by thiourea at the very beginning of oxidation reactions. Dieca inhibited the enzyme at low concentration (0.2 mM). At higher concentrations (2 or 4 mM) dieca gave a stimulation effect; the absorption of oxygen was markedly enhanced in the presence of dieca (2 or 4mM) as compared with that of the control. The increase of oxygen uptake in the presence of thiourea or dieca was not due to the direct oxidation of these reagents by the enzyme. Further studies are being carried out to make clear the nature of these effects.