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No. 4253 May 5. 1951 NATURE 725 graphy (Figs. 1-4), 20 µgm. protein being detectable. Protein degradation products do not cause a 'protein error' with the above indicators and, therefore, are not recorded on the chromatogram. Since neither bromothymol blue nor tetrabromophenolphthalein ethyl ester contains nitrogen, we are exploring the possibility that quantitative analysis of protein mixtures may be ach,ieved by nitrogen determinations of the spots. We should like to thank Dr. T. H. H. Quibell of the Chemistry Department, University of Manchester, for kindly preparing a specimen of tetrabromophenolphthalein ethyl ester.