A procedure was developed for the quantitative separation of major fermentation products of uniformly labeled glucose-C¹⁴ produced by bovine rumen microorganisms in vitro. After 45 min, the fermentation mixture was fractionated into (a) one control subsample, and duplicate fractions of (b) solid matter “precipitate“, (c) ether extract, (d) “amino acid“, (e) “sugar“, (f) CO₂, and (g) CH₄. Similar fractionation of an unfermented control sample was made. A portion of the fermentation ether extract was subjected to column chromatography to resolve (a) C₁, (b) C₂, (c) C₃, (d) C₄, and (e) C₅ fatty acids, (f) succinic, and (g) lactic acids. Each fraction was analyzed in triplicate for C¹⁴ by a direct plating technique. Corrections for geometry, self absorption, and efficiency were made by direct plating additional triplicate fraction subsamples, each containing a uniformly labeled glucose-C¹⁴ internal standard. The data were expressed as per cent recovery of added C1u. The results indicated that glucose was rapidly fermented with most of the C¹⁴ found in the ether extractable fraction as acetic acid. Significant levels of C¹⁴ were found in the “precipitate“ fractions. The data were compatible with evidence that CH₄ was derived from CO₂. The results of 6 trials indicated that there was no significant difference in the distribution of products resulting from the in vitro fermentation of uniformly labeled glucose-C¹⁴ between animals, between days within animals, or between times within days. / Ph. D.