The findings recorded in this report are concerned with the effect of chemotherapy on the occurrence of type specific pneumococcal antibodies in patients suffering from pneumonia. The incidence of the development of demonstrable specific immunity has been determined with samples of serum obtained at frequent intervals from patients with pneumonia, who were treated with either sulfapyridine or sulfathiazole. In addition to the quantitative aspects of the immunological information derived from the observations, the serological results have also formed the basis for an analysis of the significance of the presence or absence of measurable specific antibodies, in relation to the outcome of the infection in patients treated with either one drug or the other.The patients available for the immunological study presented varied clinical courses and suffered from pneumonia of varying degrees of severity. Approximately one-third of the group had bacteremia. Although the majority of them recovered promptly from the infection, some had complicating febrile episodes, with and without bacteremia, and still others died. By correlating the different kinds of clinical courses with the results of the serological tests for specific antibodies, the attempt has been made to reconstruct the course of the disease on the basis of the immunological findings. In elaborating the second phase of the problem, the study has also included clinical and postmortem findings of other patients with pneumonia who were not used for serological study, but who were serviceable in the assay of factors that appeared either to promote or retard recovery, or were associated with fatal termination.