p-Benzoquinone non-enzymatically reacted with d-glucosamine at physiological pH and moderate temperature. The reaction of p-benzoquinone with glucosamine was signaled by changes in the UV and visible spectra. The reactivity proceeded fastest at pH values above 7, with a sharp drop from pH 6.5 to 7.0, and the reaction was negligible in acidic conditions. The order of reactivity of amino sugars was d-mannosamine > d-glucosamine > d-galactosamine. From the reaction mixture, four conversion products were isolated and none was toxic to Escherichia coli even at 500-700 mu g ml(-1), while p-benzoquinone was cytotoxic to E. coli at 20 mu g ml(-1). Chitosan could react with p-benzoquinone efficiently and remove this toxicant in aqueous solution.