Recent attempts within this laboratory to demoiistratc by radioautography the specific localization of low concentrations of i.adioisotopes in rat tissues have emphasized the danger of liistochemical fogging of photographic emulsion. Boyd and Board ( '49) called attention to this phenomenon, and more recently Board ( '51) implicated the tissue sulfhydryl groups as responsible agents for this cllemical reduetion. Several investigators have considered the likelihood of misinterpretatioii of tissue radioautographs due to histochemical fogging, and have proposed methods to prevent this phenomenon, such as the coating of tissue sections with a plastic film: for example, by celloidin, Geoii Resin, or Formvar (Gross et al., '51 ; Eidinoff et al., '51). Williams ( '51) developed a dry method of inouiitiiig tissues, which was reported to prevent the chemical fogging of the emdsion. Others have used stripping film, mliich is provided with a thin base which is said to protect the emulsion, Pelc ( '47), Boyd and Williams ( '48) and RfacDonald ( '49). Holt and Warren ( '50) overcame the problem by providing ail air gap between the tissue and the emulsion. Furthermore, Boyd ('49) reported that control tissues fixed in Bouin 's solution and placed directly upon NTR emulsion caused no chemical reduction.