Abstract
1. Acid fuchsin possesses a bactericidal power, selective as between Gram-negative and Gram-positive spore-bearing aerobic organisms. The selective feature is the reverse of that of gentian violet. 2. This reverse selective activity is also possessed by simpler sulfonated substances and would appear to be dependent in some way on the presence of SO(3) radicals. 3. Selective penetration, if at all concerned in the behavior of gentian violet and acid fuchsin toward bacteria, must play a very minor rôle. 4. In the case of gentian violet the power to kill organisms and the power to prevent their growth run, so far as the selective feature is concerned, parallel. In the case of acid fuchsin, the bacteriostatic and bactericidal selective features do not run parallel. 5. Bacillus pyocyaneus, an organism resistant to gentian violet, is quite susceptible to acid fuchsin. This organism is the cause of annoying and persistent infections in wounds. The observations on the effect of the sulfonated substances may give a lead as to the control of these infections.