Reports in the literature on the fate of ingested cellulose in nonruminant animals and man present a confusing picture. Some workers (1) state that digestion, or utilization, of cellulose takes place to an extent varying from 1.5 to 97 percent of the ingested cellulose, while others (2) conclude that cellulose cannot be utilized at all. The present isotope study was undertaken in an attempt to resolve these discrepancies.Uniformly labeled C14-cellulose was prepared from defatted C14-labeled soybean meal. Protein was removed from the meal by thorough extraction, first with cold water, then with 10-percent aqueous sodium hydroxide. Hemicelluloses were further extracted with 10-percent aqueous potassium hydroxide, and the residue was washed successively with water, ethanol, and ether. The dried product (specific activity, 9060 count/min mg) was used in the feeding experiment. This material contained less than 1 percent of protein and yielded no reducing sugars after 6 hours' hydrolysis in 2N sulfuric acid at 95?C. Since cellulose is the only major non-nitrogenous constituent present in defatted soybean meal which shows these solubility characteristics and this resistance to acid hydrolysis, the material was considered to be essentially pure cellulose.Four growing male albino rats Little effect on the fluorescence of epinephrine was noted.