During the past decade a great deal of research has centered on the discovery of new and more powerful antibiotics and on their application to man's fight against bacterial infections. Among the wonder drugs studied most extensively and applied most successfully, penicillin and streptomycin hold foremost positions. The host of clinical, biological, and chemical data which has been accumulated has pointed the way to more effective use of these antibiotics against diseases caused by microorganisms and has expanded the horizon for further investigations.It is only natural that a great deal of attention should have been devoted to the manner in which these chemotherapeutic agents fight and kill harmful bacteria. Many eminent biologists and biochemists have tried to explain their mode of action, but they have concerned themselves primarily with the chemical interaction believed to take place between a given antibiotic agent and the various essential substances present in the cell which it attacks and destroys. Little if any attention had been devoted to the mechanisms whereby bacteria and antibiotic came into sufficient physical proximity, or even direct contact, until Hauser and coworkers (9) at the Massachusetts Institute of Technology noted that both penicillin and streptomycin salts, in the aqueous solutions in which they are most commonly used for parenteral injections, are present in the form of colloidal micelles. This has led to further work on these systems from a colloid-chemical point of view, in an effort to ascertain how particle size, electrical charge, capillary activity, and other properties inherently colloidal might affect the mode of action of these drugs.