A rapid, sensitive firefly bioluminescence ATP assay using a liquid scintillation counter for measurement of assay light output is described. The use of partially purified luciferase and synthetic D-luciferin in this assay permits the detection of as little as 10−16moles ATP and as few as 10 yeast cells. The assay light output is directly proportional to the quantity of ATP from at least 10 to 104fmol and is on scale throughout this range. The method developed for detection and enumeration of viable microorganisms, which requires only a few minutes per sample, involves extraction of the cellular ATP with DMSO at room temperature and estimation of the ATP by the bioluminescence assay—for dilute suspensions, prior concentration and isolation of the cells by membrane filtration is necessary. The ATP contents of a selection of yeast and bacteria determined using this technique are reported. A linear relation between quantity of ATP and number of yeast cells is demonstrated, and the changes in intracellular yeast ATP pools during fermentation of wort by ale and lager yeast are presented. A number of other possible applications of this ATP assay in brewing research, development, and microbiological quality control are evaluated and/or discussed.