The title paper in this compilation, "Filtrable Bacteria," was published in the Journal of Infectious Diseases in 1931. This was the third in a series dealing with the subject of bacterial variability. The first ("Microbic Dissociation") was published in 1927, and the second ("The Twort-d'Herelle Phenomenon") in 1928. "Filtrable Bacteria" continues the study of dissociative variations among bacteria, and addresses the method of production, the cultural behavior, the morphology and the filterability of what may be termed "the filterable virus stage" of the Shiga dysentery bacillus. PHILIP HADLEY This present compilation also incorporates a 1937 article, "Further Advances in the Study of Microbic Dissociation."