To ensure reliable manufacture of high quality beer, the brewing yeast must be maintained in a highly active state. The conventional methylene blue staining method, which is very simple but not highly sensitive, has been improved to allow sensitive measurement of yeast activity. Two kinds of yeasts with different activity levels were stained with methylene blue solution with pH levels between 4 and 12. The results showed that highly sensitive measurement of yeast activity was possible when the staining solution was alkaline. When cells were stained with methylene blue solution with a pH of 10.6 at 25 degrees C for 15 min (the alkaline methylene blue staining method), the percentage of unstained cells was consistent with viability measurements made using the slide culture method. The alkaline methylene blue staining method makes possible sensitive, quick, and simple measurement of yeast activity, which is essential to the management of fermentation process.