The physiological state of a yeast population used for inoculation determines how rapidly the cells adapt to new environmental conditions, begin proliferating and utilising extract. The decision as to whether a yeast culture is suitable for re-pitching should not be based only on viability determinations since this can be misleading. Increased proteolytic activity in a yeast population indicates the onset of senescence. A flow cytometric method has been developed for measuring a wide variety of proteinases in Saccharomyces cerevisiae employing a commercially available casein-dye conjugate. The detection of intracellular proteinase activity gives an early indication of apoptotic events and allows improved assessment of the physiological state of a yeast population. This knowledge will assist the industry to optimize the selection of yeast and its subsequent fermentation performance. Yeast cell autolysis with all its negative consequences for beer quality and stability will thus be minimised.