The more important criteria for identification of the S form of H. pertussis and for its differentiation from R variants are presented in summary fashion in Table IV. The differences as indicated in detail in the foregoing sections and as shown briefly in this tabulation are so clear-cut that they call for little, if any, further comment. Of all the differential characteristics, the morphological, the serological, and the cataphoretic seem to be the most distinctive, and of these, the cataphoretic, in our hands, has been the most conclusive. It would appear from the foregoing results that all recently isolated strains, provided that they are grown upon suitable media, fall into a single uniform serological type. This is true whether the strains are isolated from cases of whooping cough in Europe or in any part of this country. It is also apparent that when they are subcultured upon laboratory media deficient in fresh blood, dissociation occurs with the appearance of morphologically, culturally, and antigenically different variants. In keeping with the current classification of bacterial variants, we feel that the uniform recently isolated strain should be designated the S form and the laboratory variants the R form of H. pertussis. Whether the R variants fall into sharply defined phases as pointed out by Leslie and Gardner, remains to be confirmed. Lawson and the writer have never noted their "Phase II" and Toomey (21) is in agreement with us that sharply defined, mutually exclusive subvarieties of R variants probably do not exist. We are undertaking further studies of this somewhat mooted point. Dawson, as a result of detailed studies concerned with the dissociation of pneumococci (22) and of streptococci (23) has shown that these organisms have three variant forms and he proposes that the terminology currently employed for pneumococcal variants be changed to conform with the terms used in the description of corresponding variants of other bacterial species. The first form, encapsulated, at present called S, he designates mucoid (M); the second, at present R, he calls smooth (S); and the third, a new and distinct, grossly rough variant which he describes for the first time,l he would call rough (R). Hadley (24) has found that most organisms show these three chief colony forms. From conversation with Dr. Dawson, it seemed that our S which is encapsulated and has a moist mucoid colony has the character of his M form. Whether the subvarieties of R variants (III and IV of Leslie and Gardner, A and B of Mishulow) correspond to his S and R, remains to be seen. For the time being it has seemed more practical to use the terms S and R as generally employed. As we study H. pertussis variants further, it is possible that these three chief colony forms, constituting the usual pattern for other bacterial species, may emerge as well defined types. The finding that H. pertussis when first isolated is a single specific serological type, in S form, and that this antigenic phase may be maintained by suitable cultural management has certain definite implications. One bears upon further transmission experiments directed toward the establishing of the etiological relationship of the organism to whooping cough. Working with monkeys, Sauer (25) produced suggestive manifestations of the disease in 8 out of 76 attempts. It is very probable that his failures may be laid to the use of R forms as inoculum. Criticism directed against the conclusiveness of the occasional successful transmission experiment (Sauer (25), Rich et al. (26), MacDonald and MacDonald (27)) with freshly isolated H. pertussis put forward by those entertaining the combined H. pertussis and filtrable virus theory is being met in the experiments already reported upon (4) and still being carried out in this laboratory, by subculturing the theoretically pathogenic S form long enough to preclude the presence of virus. A second implication of importance related to the preparation of H. pertussis vaccines. Madsen (28), Sauer (29), and Frawley (30), the last worker using Krueger's specially prepared H. pertussis vaccine (31), have recently reported favorably regarding protection against whooping cough by means of suitably prepared vaccines. Sauer following the lead of Madsen insists upon the use of freshly isolated organisms; Krueger's special vaccine is made from similar strains. The ready identification of the S form of H. pertussis and the practicability of its maintenance brought out above puts the preparation of antigenically effective vaccines upon a sound basis. In cooperation with Dr. J. A. Doull of the Department of Hygiene and Bacteriology and Dr. H. J. Gerstenberger of the Department of Pediatrics, we have begun a carefully controlled study of the prophylactic value of H. pertussis vaccines made from organisms shown to be in the S form according to the criteria outlined above. By the same token, the therapeutic value of H. pertussis vaccine in active cases of the disease can be determined only after carefully controlled studies have been carried out with such antigenically effective preparations.