1. Three hundred and two samples of sera from normal people, and 847 samples from cases suspected of enteric, or from contacts, have been examined for heat labile agglutinins against B. typhosus and allied organisms.

2. Sera from the “not normals” show a higher percentage of agglutinins against nearly all the organisms tested than sera from the “normals.”

3. In the contact group of the “not normals” the incidence of agglutinins is practically the same as in the “normal” group.

4. Sera from inoculated persons show a higher percentage of agglutinins against those organisms included in the T.A.B. vaccine used during the late war, than the sera from non-inoculated persons.

5. There is a higher percentage of agglutinins for B. typhosus, B. paratyphosus A and B. paratyphosus B among males than among females, especially in the age groups 20–30 and 30–40.

6. The proportion of normal males whose sera agglutinate B. typhosus, B. paratyphosus A and B. paratyphosus B and the group suspension is significantly lower than corresponding proportion noted in an investigation recorded by Rosher and Fielden in 1922. The proportion of agglutinating sera for these strains among normal females is substantially the same in our findings as in theirs.

7. The associations between ability to agglutinate different pairs of organisms are shown to be statistically significant in those cases which might arise from T.A.B. inoculation.

8. For these reasons it seems likely that the present distribution among the adult male population of agglutinins for B. typhosus, B. paratyphosus A, B. paratyphosus B and for Salmonella strains allied to B. paratyphosus B when in the group phase, was largely determined, in the period 1925–6, by the results of T.A.B. inoculation carried out during the war. It is probable that this effect is slowly disappearing.

9. Statistically significant associations were found between certain other pairs of organisms, which could not be explained as resulting from inoculation with T.A.B. vaccine, or as a result of any common antigenic stimulus. These associations were found mainly in the “not normal” group; and their meaning, at the moment, remains obscure. It may be noted that the number of sera agglutinating these particular pairs of suspensions was, in most cases, small.

10. Our results, so far as they yield any information on this point, do not suggest that infection with any one of the organisms studied by us increases the probability of the patient's serum agglutinating some other bacterium, with which the infecting organism is not antigenically related.

11. High titres were found chiefly in sera from the “suspected of enteric” group. The normal group usually gave low titres.