The monosaccharide content of the ethanol-soluble moiety of crude polysaccharide fractions of the tapeworm Hymenolepis diminuta and the blood fluke Schistosoma mansoni was investigated by gas-liquid chromatography of alditol acetate derivatives. A prominent, unusual peak was found, and mass spectrometry suggested identification as an aminotetrose, a fourcarbon aminosugar reported only once in the past 15 years [D. A. Cumming C. G. Hellerquist, and O. Touster (1981), J. Biol. Chem. 256, 7723–7726]. The unknown molecule was found to be tris(hydroxymethyl)aminomethane (Tris), whose acetylated derivative has a mass spectrum identical to that of an aminotetrose. The source of the Tris was traced to buffers used in conjunction with polysaccharide isolation.