Since the time of Ehrlich, chemotherapy has been largely concerned with efforts to fortify the weaknesses of bactericidal and paracitocidal substances by chemical manipulation of the molecule. It has been recently found, however, that the end in view may, in some cases, be easily achieved by the use of simple mixtures of certain dyes which have been previously shown to possess opposite selective bacterio-static properties. The fact was reported to this society last year that acid fuchsin possesses a selective bacteriostatic power which is in many important respects the opposite of the selective power possessed by gentian violet. Acid fuchsin and gentian violet will not, however, mix. I have recently shown that neutral acriflavine possesses, like acid fuchsin, a reverse selective bacteriostatic power; and acriflavine and gentian violet will mix. Moreover the resultant substance acts as a mixture. It is thus possible to fortify the weakness of each of these two dyes with the strength of the other. If a mixe...