Bacteriological diagnosis of the Enterica is only established with certainty on recovery of the infecting organism by recognised laboratory methods. In default, recourse must be made to a serological investigation into the agglutinating power of the patient's serum, when, theoretically, little difficulty ought to be experienced. In enterica suspects, unvaccinated or uninfluenced by previous enterica disease, demonstration of agglutinins reacting with the Bact. typhosum, paratyphosum A, B, or C in relatively high dilutions of their sera should be considered adequate evidence. In the vaccinated, elevation of agglutination titre—as determined by repeated examinations during the course of the disease—has been generally accepted as criterion for existence and type of infection. The principle of ascending dilutions, on which such “quantitative serum analysis” is based, is explained by the fact that in an infection with one organism a specific agglutinin is formed for that particular organism and group agglutinin is for other organisms closely related biologically; the specificity, therefore, of the agglutination reaction must be wholly dependent on the basis of dilution, the specific agglutinin being present in greatest quantity and operative in a range much exceeding that of the secondary group agglutinins. Reasonable as this view may seem in principle, it is far from easy of application in practice. Lack of accurate information as to the day of disease on which serum was taken, unreliable case histories in regard to vaccination or previous enterica infections, and inability of private practitioners to induce patients to submit to repeated blood collection constituted here primary difficulties.