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SUMMARY: Strains of Ruminococcus albus were isolated from sheep fed lucerne chaff once daily. Antisera to these bacteria were prepared in rabbits and labelled with fluorescein isothiocyanate. A specific labelling reagent was produced from these conjugates by chromatography on Sephadex G-25 and two absorptions with mouse liver powder. The Breed smear technique was adapted for counting fluorescent cocci. Background fluorescence in rumen fluid samples was controlled by making suitable dilutions. A sample taken 10 hr after feeding had a mean value of 5.76 (s. e. 1.16) x 106 cocci/ml. Samples taken from different parts of the rumen at the same time showed random but significant variation in the number of fluorescent cocci/ml. The natural fluorescence of some particles of digesta was bright enough to obscure the specific fluorescence and attempts were made to release fluorescent cocci by homogenization; this was partially successful, and the average ratio of untreated to homogenized counts was 1:1.17. Diurnal variations in the number of cellulolytic cocci were followed over a 4-day period. Minimum numbers were recorded 6 hr after feeding and maximum numbers 18 hr later. There was a measurable day-to-day variation.