There are many pretreatment and detection methods for divalent cations in blood. But our purpose was to study the pretreatment of blood for the determination of cations by Ion Chromatography. We compared with recovery of Mg$^{++}$, and Ca$^{++}$ contained in plasma according to four pretreatment methods, that is, add of trichloroacetic acid and perchloric acid, dilution with distilled water and membrane filter method. As a result, add of trichloroacetic acid was found to be the most suitable method for good recovery of Mg$^{++}$ (98.0%) and Ca$^{++}$(96.0%) in plasma, and the Mg$^{++}$ and Ca$^{++}$ contents in plasma was 20 and 102 ($mu$g/ml)