cal usefulness of a color reaction given by with the diphenylamine reagent of Dische. It has been shown that the purple color produced in this reaction is probably due to the presence of a deoxyhexose structure in the sialic acid molecule (5). Sialic acid, or a closely related substance, appears to be a common constituent of mucoproteins (glycoproteins) including orosomucoid of (6, 7). The color produced in the diphenylamine reaction reflects the concentration of the mucoproteins in the being tested. In our experience, and as reported by Anderson and Maclagan (8), the color developed with diphenylamine appears to be a specific reaction of mucoproteins. An elevation in the mucoproteins found in the alpha globulin fraction of is characteristic of many diseases (7). As pointed out in a previous report (4) a constant feature of the diphenylamine reaction was the concomitant rise in the color index of the serum blank associated with the increased color intensity of the samples tested. The serum blank in this instance was the combination of deproteinized with the sulfuric acetic acid mixture which did not contain diphenylamine. This report will consider the chemical and clinical aspects of the serum blank reaction. It will be shown that the color produced when a mixture of sulfuric and acetic acids is heated with the deproteinized sample is also probably due to sialic acid. The simplicity of the test suggests its usefulness as a new method for measuring the intensity of inflammation.