1. Agglutinins are present in the blood of patients suffering from lobar pneumonia during some stage of the disease in a large percentage (73.8 per cent.) of the cases due to pneumococci belonging in groups I, II, and IV.
2. In most very severe and fatal cases agglutinins cannot be demonstrated, and it is probable that their absence during the later days of the disease may have unfavorable prognostic significance.
3. No agglutinins are demonstrable by the technique employed in the blood of patients suffering from infection with Pneumococcus mucosus (group III).
4. In certain cases agglutinins may be demonstrable for only one day, and in other cases they may persist for several weeks.
5. When agglutinins are demonstrable they usually appear at about the time of the crisis.
6. It has not been possible to demonstrate that treatment with immune serum has any effect on the appearance of agglutinins.
7. The agglutinins present in cases due to organisms of types I and II are always specific for the type of organism causing the infection. In certain cases the agglutination reaction may be more active or appear earlier when the homologous organism is employed than when other organisms of the group are used in the test. In cases due to organisms of type IV, the serum never causes agglutination of any organism except the homologous one.