By means of the alexin-fixation reaction it has been found possible to demonstrate consistent antigenic differences among some strains of B. typhosus. In accordance with these differences a number of strains have been tentatively allocated in three fairly well-defined groups. Group I is made up of strains of rather recent isolation; no strain in this group has been under artificial cultivation for more than two years. Older strains, isolated from three to fifteen years ago, compose Group II. The third group is made up of those strains that fall neither in Groups I nor II, and that cross-fix irregularly with each other and with the members of the other groups. Group III strains are all more than three years old.

By means of agglutinin-absorption experiments, findings have been obtained that, in general, harmonize strikingly with the results of cross-fixation. This confirmatory evidence, secured by a widely different serologic method, tends strongly to prove that these antigenic differences among typhoid strains, as demonstrated by alexin-fixation, represent an actual condition and are not fortuitous.

It is considered that the evidence of antigenic differences thus far discovered among different strains of B. typhosus is sufficiently valid to warrant the presentation of these data, and sufficiently encouraging to justify extension of this work, especially with regard to the comparative protection afforded by sera obtained by immunization with strains of different character.

The facts that a serum immune to any recently isolated strain cross-fixes with all other strains, old or young, while sera immune to older strains do not so cross-fix, would lead to the seemingly justified assumption that only do the young strains contain all of the antigenic complexes of typhoid bacillary protein. The logical conclusion, then, would be that a young strain ought to afford the most efficient protection when used for prophylactic immunization against typhoid fever. However, the results of the work on agglutinin-absorption, if correctly interpreted, do not warrant the assumption that a univalent vaccine is sufficient. Therefore, although the grouping of strains here given is but tentative, it seems advisable, for the present, to recommend the use of a balanced polyvalent typhoid vaccine, for immunizing and therapeutic purposes, compounded in accordance with these groups. The experimental work here set forth distinctly does not uphold the prevailing practice of employing a single old strain for prophylactic immunization against typhoid fever.

It is a pleasure here to acknowledge my gratitude to Dr. F. P. Gay for the helpful interest he has taken in this work.

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