DURING the course of our investigations on the biosynthesis of 3,6-dideoxy-hexoses, found in the endotoxins of Salmonella and Escherichia 1–3 and in the eggs of Parascaris equorum 4, the need arose for a sensitive and specific assay. A consideration of the structures of these compounds suggested to us that the malonaldehyde–thiobarbituric acid reaction, which was adapted by Waravdekar and Saslaw5 for the estimation of 2-deoxy-ribose, might equally be applicable to the quantitative determination of 3,6-dideoxy-hexoses, and indeed, of 3-deoxy sugars in general. A sample of ascarylose (3,6-dideoxy-L-arabino-hexose), which was the generous gift of Dr. E. Lederer, was used to test this hypothesis.