The Nelson–Somogyi assay has been widely used to measure reducing sugars in plant research. We found that the L-amino acids cysteine, cystine, serine, tryptophan, and tyrosine (at 0.1 to 1 mM) react in the Nelson–Somogyi glucose assay, yielding high absorbance values. Alanine, arginine, aspartic acid, glutamic acid, glycine, histidine, hydoxyproline, isoleucine, leucine, lysine, methionine, phenylalanine, proline, threonine, and valine showed little or no effect on color development. Manganese ions strongly inhibited color development in the presence of glucose, serine, cystine, and tryptophan. Investigators measuring glucose in biological materials using this assay must consider the activity of amino acids and (or) the effect of manganese ions in the reaction mixture when interpreting their results. Key words: Nelson–Somogyi assay, sugar analysis, glucose assay, reducing sugar assay.