The significance of pneumococcal types is discussed in this report. Since my initial report on the distribution of pneumococcal types in 150 cases of lobar pneumonia from 1920 to 1922, I have continued to study this subject. From January 1922 to March 1927, additional data were collected. Table I shows the results in two series and compares them with previously published data. The main point of interest is the progressive decrease in cases of pneumonia caused by Type II pneumococcus. Most cases occurred in the Smethwick district, and the figures may indicate a real local decrease of Type II and an increase of Group IV cases. However, the isolation of a Group IV strain from sputum does not prove it caused the disease. The cultures of pneumococci were derived from sputum, and some samples were obtained when the disease had been in progress for some time. In my report (1922), I described instances where several serological varieties of pneumococci were found in the sputum of a pneumonia patient. One instance was particularly striking, where the sputum, No. 112, taken on the sixth day of the disease yielded a Type I culture and three strains of Group IV. On other occasions, different specimens of sputum from the same case, taken at varying periods after the onset of pneumonia, were found to contain two or more serological types. Three alternative explanations are possible: the patient was a carrier of several Group IV strains and became infected with a Type I strain; the patient was a carrier of a Group IV strain and a mutation occurred in his air-passages; or the Group IV strains might be derived from the Type I in the course of successful resistance against the latter strain. Further experiments were made to determine the causal connection of the Group IV strains with the pneumonia. The results showed that the Group IV strain produced large capsules in the blood of the mouse and a peritoneal washing reacted vigorously with the type serum. The Pn. 160 culture appeared frequently in the course of the investigation. The report also discusses the production of attenuated strains of pneumococci. Attenuated strains can be produced by growth in immune serum or on solid media. The R form of the pneumococcus is recognizable by the morphological appearances of the colonies. The R form can be obtained from virulent cultures by growth on chocolate blood agar. The R form can also be obtained from virulent cultures by growth in immune serum. The R form can revert to the smooth form in the mouse. The ability of an attenuated R strain to revert to the virulent S form persists during prolonged periods of subculture in the R form. The subcutaneous inoculation of a mass of culture under the skin furnishes a nidus in which the R pneumococcus is able to developThe significance of pneumococcal types is discussed in this report. Since my initial report on the distribution of pneumococcal types in 150 cases of lobar pneumonia from 1920 to 1922, I have continued to study this subject. From January 1922 to March 1927, additional data were collected. Table I shows the results in two series and compares them with previously published data. The main point of interest is the progressive decrease in cases of pneumonia caused by Type II pneumococcus. Most cases occurred in the Smethwick district, and the figures may indicate a real local decrease of Type II and an increase of Group IV cases. However, the isolation of a Group IV strain from sputum does not prove it caused the disease. The cultures of pneumococci were derived from sputum, and some samples were obtained when the disease had been in progress for some time. In my report (1922), I described instances where several serological varieties of pneumococci were found in the sputum of a pneumonia patient. One instance was particularly striking, where the sputum, No. 112, taken on the sixth day of the disease yielded a Type I culture and three strains of Group IV. On other occasions, different specimens of sputum from the same case, taken at varying periods after the onset of pneumonia, were found to contain two or more serological types. Three alternative explanations are possible: the patient was a carrier of several Group IV strains and became infected with a Type I strain; the patient was a carrier of a Group IV strain and a mutation occurred in his air-passages; or the Group IV strains might be derived from the Type I in the course of successful resistance against the latter strain. Further experiments were made to determine the causal connection of the Group IV strains with the pneumonia. The results showed that the Group IV strain produced large capsules in the blood of the mouse and a peritoneal washing reacted vigorously with the type serum. The Pn. 160 culture appeared frequently in the course of the investigation. The report also discusses the production of attenuated strains of pneumococci. Attenuated strains can be produced by growth in immune serum or on solid media. The R form of the pneumococcus is recognizable by the morphological appearances of the colonies. The R form can be obtained from virulent cultures by growth on chocolate blood agar. The R form can also be obtained from virulent cultures by growth in immune serum. The R form can revert to the smooth form in the mouse. The ability of an attenuated R strain to revert to the virulent S form persists during prolonged periods of subculture in the R form. The subcutaneous inoculation of a mass of culture under the skin furnishes a nidus in which the R pneumococcus is able to develop