1. Of eighty-six strains of Haem. influenzae isolated from the naso-pharynx of normal persons, sixteen had the general characters of Pittman's “smooth” type.

2. These sixteen strains and one “smooth” strain isolated from a case of meningitis were submitted to various serological tests, including the separation and partial purification of a carbohydrate fraction from the meningeal strain and from four of the sixteen strains.

3. By precipitin tests carried out with these purified fractions, it was possible to identify three of the five “smooth” naso-pharyngeal strains and the meningeal strain, as belonging to Pittman's type e, and one of the “smooth” naso-pharyngeal strains to Pittman's type a.

4. Some indication of grouping, within this small sample of smooth strains, was obtained by various other methods, such as precipitin tests carried out with simple saline washings or agglutination reactions; but, apart from the four type strains a, b, e and f, received from Miss Pittman, no strain could be satisfactorily typed by any of these methods. Only by using partially purified polysaccharide fractions was it possible to assign any of the “smooth” nasopharyngeal strains to their correct type.

5. The testing of these naso-pharyngeal strains against antisera prepared against types a, b, e and f by direct agglutination, showed that either many of these strains contained some proportion of the smooth antigens, or that the antisera contained antibodies acting on other antigenic components, although, in relation to the type strains themselves, they appeared to be specific.

6. The examination of the eighty-six strains, as a whole, revealed the extreme antigenic heterogeneity that has been noted by many previous workers.