PURPOSE: To determine the number of facterial cells in a solution, easily, rapidly, and accurately, by concentrating the cell solution, and measuring the respiratory activity of the bacterial cells which is proportional to the number of the cells.

CONSTITUTION: A specimen solution which is to be subjected to the determination of the number of bacterial cells, is maintained at 0W5°C for 30W60min, and filtered with a membrane with does not permeate the cell. The membrane 1 is held near the Teflon diaphragm 5 with a holder 2 facing the surface attached with the cells toward the Teflon diaphragm 5 covered with the cathode 4 of the oxygen electrodes. The oxygen electrode is immersed in a phosphoric acid buffer solution containing a nutrient source such as glucose at a specific concentration and saturated with oxygen. About 10min after the application of a definite voltage to the oxygen electrodes, the stationary electric current is measured, and the number of objective living cells can be determined from the level of the current. The method enables the accurate determination of the number of the living cells in a dilute solution containing as low as ≤105/ml of living cells.