Using 10 microns thick cryostat-slides of spleen and gut from white rats the following substances (in 2 different concentrations) were tested as substrates for the enzyme phenoloxidase compared with dihydroxyphenylalanine: pyrogallic acid, resorcin, 2.4-dinitrophenol, tyraminhydrochloride, o-cresol, and epinephrine. Only pyrogallic acid and resorcin resulted in positive reaction. Resorcin showed a positive reaction only when the higher concentration is employed. The investigations show clearly that the detectable enzyme in gut, spleen, and oral cavity is a polyphenoloxidase, which occurs consequently not only in granulocytes.