Although bacteriophage was discovered some twenty-odd years ago, little is yet known of its real nature. It has been investigated extensively, but is still best known by its notion on micro-organisms susceptible to it. When a small amount of bacteriophage is added to a young culture of susceptible organisms it brings about their dissolution, or lysis within a few hours. The time required for this phenomenon is variable, but the importance of the result is readily recognized and was immediately of great interest to medical men. At first it appeared that in this substance had been found a “cure-all” for all bacterial diseases, and it has been from the therapeutic as well as the physico-chemical standpoints that it has been investigated. The subsequent work with this material did not bear out the glowing reports of D’Herelle. The results from its therapeutic use were so inconsistent that it was not long before its use had been generally discarded. Many explanations were offered for the variable clinical results. Although D’Herelle (8) states that the body fluids do not inactivate phage, Colbin (5 & 6) reports to the contrary. The idea of greater specificity of the phage to the susceptible organism has also been advanced, but very little work has been done to demonstrate the degree of specificity that does exist. In this work, the author has assumed that a specificity comparable to serological specificities does not exist and has endeavored to show this with different strains and species of the gene Escherichia. The proof of a species or strain specificity in this germ would warrant the hypothesis that such specificity does also exist in other pathogenic genes and thus a better explanation for the therapeutic inconsistencies resulting from the use of commercially prepared phages might be advanced. / Master of Science