Studies of microbic dissociation have usually taken as the point of departure either some deviation from the familiar colony type of the culture under investigation or instability of emulsions in normal salt solution. Variants have usually been, and still are, spoken of chiefly as “rough” or “smooth” even though abundant evidence has accumulated that in colonial morphology the variant culture may be indistinguishable from the original culture, yet differing profoundly in serological, morphological, or biochemical properties. Conversely, rough colony variants may manifest complete serological identity (Krumwiede, Cooper and Provost (1) and Savage and White (2)) with the original smooth colony culture. Perhaps the “R” and “S” terminology is the most convenient one now available but its inadequacies are constantly encountered in investigations of the phenomena of microbic dissociation. In this paper and in those which follow we have preferred to designate the original culture, from which both “smooth” and “rough” variants have been derived, as the O-culture.