It was found possible by solution in sulphuric acid to convert the new base into one which did behave as the base obtained from commercial neutral red. and also gave the characteristic vital staining of cells from two varieties of Amoeba and from sarcoma 37 ascites tumour when it was converted to its hydro -chloride, but no attempt was made in this first report to arrive at an explanation of the true nature of the new compound. Now, however, as a result of further consideration of the problem, together with that of some experimental facts already known, but with which the authors were unacquainted at the time, I have formulated the following possible explanation :By analogy with 1- and 2-phenazinol, which have been shown to be tautomeric, respectively, with l(5H)-phenazinone, and 2(10H)-phenazinone3, it seems highly probable that equivalent systems exist with 1- and 2-aminophenazine : Following the nomenclature of the corresponding keto compounds : it is suggested here that II might be called l(5H)-phenazinimine, and III, 2(10H)-phenazinimine, as shown above. Just as Badger, Pearce, and Pettit have stated3 that the system 1-phenazinol l (5H)-phenazinone must exist primarily, if not entirely, in the hydroxy form, due to hydrogen bonding with the nitrogen atom in the 10-position, so, too, it is probable that the amino form predominates in the system 1-amino-phenazine-2 (5H)-phenazinone, even though the predominance would not be expected to be as great. For the sake of academic completeness, however, both the 1 -aminophenazine and the 2-aminophenazine systems are illustrated here.The tautomers of the unsubstituted phenazinol-phenazinone systems apparently are incapable of independent existence, as by analogy are probably those of the unsubstituted aminophenazine phenaz-inimine systems also. However, appropriate substitution has made it possible to isolate both forms in one system, at least, that of l,3,4-trimethyl-2-phenazinol-l,3,4-trimethyl-2 (10H)-phenazinone4. Hence, in view of the demonstrated fact that separable isomers of 2-aminophenazine do exist when the compound is further substituted by an amino or hydroxyl group in the 8-position1, it seems a reasonable hypothesis that their structures are wholly analogous to those of the phenazinol-phenazinone systems. If this be true, the two forms of the colour base of neutral red must be as shown below : Existence of a third compound, arising from the high er-temperature neutralization of the sulphuric acid solution of the non-staining form of the base, as is likewise mentioned in the paper by Vivian and Belkin1, may possibly be due to polymerization under these conditions, but no definite conclusions can be drawn without further work.