The present paper is the outgrowth of work begun by one of us1in 1919 on the effect of the surface tension of the culture medium on bacterial growth. In a series of papers published since then it has been shown that bacteria may be divided into two main groups with respect to their behavior toward surface tension depressants: one group, which is represented by the colontyphoid organisms, grow well in mediums of low surface tension, while the other group, represented by pneumococci, streptococci and the tubercle bacilli, are extremely sensitive to slight lowering of the surface tension of the medium. In fact, members of the latter group are rendered nonpathogenic by exposure for short periods of time to proper surface tension depressants, such as sodium ricinoleate. In attempting to find the most suitable surface tension depressant for biologic work, it was found that sodium ricinoleate served this purpose