A study was undertaken to evaluate the Mann's dye method for determining the magnesium content in citrated blood plasma. It was found that blood serum and heparinized blood plasma gave similar values when measured by atomic absorption spectrophotometry and by the Mann's dye method, but that magnesium values were reduced in plasma that had been prepared with dried ACD solution. The reduction was directly proportional to the amount of dried ACD solution present. An additional dilutional effect was noted when liquid ACD solution was used. Apparently a magnesium citrate complex was formed that was sufficiently insoluble in ethyl alcohol to interfere with the color development. It is concluded that the magnesium content of citrated blood plasma determined by this method represents total plasma magnesium minus the interference from citrate. The Mann's dye method is therefore unacceptable for determining the magnesium content in liquid plasma prepared in ACD solution.