Add 8 ml. H2O and 4 ml. 10% tri-chloracetic acid to 4.0 ml. plasma or serum, let stand 5 min., and filter. To 8 ml. of the clear filtrate, add 1 ml. of gum ghatti soln. (1 g./1 1. H2O); 1.5 ml. titan yellow (0.05%) and 2 ml. NaOH (4 N). The minimum transmittance of the magnesium-dye lake is at 5250 A. The color is stable for 3 hrs. and may be carried out in presence of Ca or oxalate. Recoveries are of the order [plus or minus]0.05 mg./100 ml.