Simple clinical methods that furnish accurate data are desirable in the study of renal function, particularly for the physician to whom the complicated methods of analyzing the blood are not available. The method reported in this article is of interest as an accurate index to the urea of the blood. The standard determination of blood urea by the urease method is an admirable test but requires very careful technic, certain laboratory facilities and is somewhat time consuming. The method herein reported requires only elementary laboratory knowledge and very simple equipment; only about fifteen minutes are required, and it gives adequate clinical results. It is the estimation of the mercury combining power of deproteinized blood. NOTES ON THE LITERATURE Liebig1 in 1853 made use of the ability of mercury to combine with nitrogenous products in a method for the determination of urea in urine. In 1921 Friedländer2 reported a titrametric method