TETRAZOLIUM salts have found various uses in both plant and animal tissues, and in the past six years more than seventy papers as well as several reviews1 have appeared. But one of the most commonly used of these salts, 2,3,5-triphenyl tetrazolium chloride, can be reduced in various ways, as, for example, by certain naturally occurring compounds carrying—SH groups, if the pH 2 is greater than 9, by glucose3 at temperatures exceeding 40° C. and by water4 if exposed to strong radiant energy of wavelengths shorter than 450 mµ, especially at high pH 5. Also extracted dehydrogenases and oxidases have caused reduction of triphenyl tetrazolium chloride in vitro (see, for example, ref. 6), and reducing agents like sodium hydrosulphite reduce it readily. Radiation can also cause some other curious effects, which I wish to report briefly.