1. Gramicidin and tyrocidine depress the surface tension of aqueous solutions. Tyrocidine is more active in this regard than is gramicidin. 2. The ability of gramicidin to depress surface tension is improved by the addition of organic solvents such as glycerin which increases the solubility of gramicidin. 3. Serum decreases the activity of tyrocidine less than it does the activity of gramicidin, sodium oleate and aerosol OT. 4. The bactericidal and hemolytic effects of gramicidin are destroyed by heat but its property of altering surface tension is heat stable.