1. The Schwartzenbach method has been adapted for the simultaneous determination of calcium and magnesium in serum and spinal fluid. The method appears to be adaptable to all body fluids, including thoracic and abdominal transudates and exudates. Its use in the clinical laboratory is recommended because of the great saving of time, particularly in the determination of magnesium. 2. A series of 100 pathologic cases of various clinical entities has been analyzed by the new method and checked with the standard methods. A calculation and statistical analysis has revealed the Mg/Ca molar ratio to be of possible clinical significance.