A method is described for precipitating a reducing organic phosphate from the trichloracetic acid filtrate of frog muscle. The precipitated substance, determined by reducing power, is considered as "lacta-cidogen." It is much smaller in amount than the lacta-cidogen originally defined, as recent studies of phos-phocreatine and pyrophosphate would imply. The reducing values of the precipitate called "lactacidogen" are similar in the same right and left muscles. When fixed in contraction in liquid air, the reducing values are less than when fixed uncontracted, as occurs 30 sec. after a 5 sec. tetanus. The decreased reducing values in muscles fixed during contraction are considered to show "lactacidogen" decomposition during contraction.