The precise mensuration of the intracellular cation content is very significant from the biological point of view. When a salt concentration is medium is higher than in bacterial cells, it is very difficult to measure the intracellular cation content because of extracellular high contamination. In such a case, there is no other way except repeated washing for elimination of the outer contaminants. With the repeated washing method, authors attempted to measure accurately the intracellular cation content of Staph. aureus 209 P, cultivated in the medium of the various degrees of salt concentration, by flame photometory and atomic absorption spectrophotometory. The results were as follows. 1. When univalent cation concentration in cells is lower than in medium, it is necessary to wash the cells several times. The washing effect on extracellular contaminants could agree with the theoretical equation of washing that the contaminant decreased exponentially. The successive decrease of assay values by an easy grade must show a leakage of the intracellular Na. The reflective point could be considerated to show the Na content. 2. As in general potassium concentration in cells is higher than in medium, the accurate value was decided by extrapolation of the linear analytical quantity vs. washing time to zero time. 3. In spite of the extreme variation of sodium salt concentration in culture medium, the univalent cation content in Staph. aureus 209 P had hardly any shifts. 4. Bivalent cations, Mg and Ca, were not affected by the washing.