The method involving cold digestion, extraction with dithizone, decomposition of the mercury dithizonate by heating, and estimation of the mercury vapor produced by ultraviolet photometry has been modified so that only 0.1 ml of blood is required. The method has also been modified by use of an electric furnace instead of a Bunsen burner combustion chamber. With the aid of these modifications concentrations of mercury in the millimicrogram (nanogram) range can be determined.