1. Pneumococci were obtained at one time or another from the nasal passages or throats of 80 per cent of 105 adults and children studied. In adults, they were obtained more frequently from the throat; in children, as often from the nasal passages as from the throat.
2. Of 500 pneumococcus strains studied, 97 per cent proved to be serologically specific. They formed smooth colonies and were for the most part avirulent for mice. Types I and II were obtained from one and two individuals respectively on one occasion only. Type III was obtained from nine individuals; Type XIII from nine individuals; Type XVI and Type XVIII from three individuals, for varying periods in each case. Atypical pneumococci were secured from 13 persons on single and scattered occasions. They varied in colony morphology, did not kill mice, or agglutinate in saline, but flocculated in all types of antipneumococcus sera employed and over a wide pH range in acid buffers. Their occurrence was apparently not associated with any type-transformation or virulence-enhancement process in vivo.
3. Strains of pneumococcus obtained on successive cultures from a given carrier were, with rare exceptions, of the same serological type and were similar in colony morphology, virulence for mice, and other tested biological characteristics.
4. Pneumococci of Types I and II were obtained under conditions suggestive that they lacked a capacity to spread readily; pneumococci of Types III and XIII, on the other hand, were obtained under conditions suggestive that they were spreading from person to person.
5. The persons studied differed consistently with respect to the occurrence of pneumococci. Some were pneumococcus-free, some were transient carriers, some periodic, and some chronic carriers. Data are given which suggest that the differences were due to variations in host resistance.
6. The incidence of pneumococci in all individuals studied underwent a seasonal variation paralleling that of coryza and sore throats in the same persons.