MORRIS1 has recently described the use of new reagent (0·2 per cent anthrone, a reduction product of anthroquinone, in 95 per cent sulphuric acid) for quantitative determination of carbohydrates. We have found that it can be used not only for distinguishing streptomycin B (a mannoside) from streptomycin A, but also for estimating the amount of the former present in a mixture. The glucosamine moiety, present in both molecules, apparently does not react with the reagent. Results obtained are in accord with those calculated from biological and chemical assays, making the accepted assumption about the relative biological activities of the two streptomycins.