Two strains of B. dysenteriae isolated from cases of asylum dysentery, were found to be readily agglutinable with stock Flexner-Y serum but serum prepared from the one had only a feeble action on the other. Absorption experiments revealed a like specificity. They differed in certain particulars in their action on certain carbohydrate media, and the attempt was made to obtain, by various procedures, variants from each strain giving fermentation reactions as alike as possible.

In the course of this work, an interesting and probably hitherto undescribed type of variation was met with, viz. the capacity exhibited by one strain which fairly promptly fermented the substance employed (isodulcite), of throwing off variants having no action on the substance. These variants appeared as white papillae on the surface of the original red ancestor and the description and analysis of this “reversion” process form the main interest of this paper. Though a fairly close approximation in fermentation characters was realised by selection of variants from each of the original strains, little or no approximation was apparent in respect of the serological affinities of these final variants.