Pneumonia can be produced in mice, which have not been previously prepared, by intranasal inoculation of broth cultures of certain strains of pneumococci. Lesions which are quantitatively different can be produced in different breeds of mice by inoculation of the same type of pneumococcus. Similar inoculation of different types of pneumococci into one breed of mice results in lesions which are qualitatively different. In general, these lesions are as follows: a diffuse pneumonia and an acute glomerular nephritis in unselected mice receiving Type I strains; a confluent pneumonia and a tubular nephritis in the case of Type II strains; and as result of Type III strains, an interstitial pneumonia with extensive gelatinous pleurisy, together with necrosis and abscess formation in the spleen and cervical lymph nodes. Resistant strains of mice with Type III pneumococci show slight changes in the lungs, but marked lesions in the spleen and cervical nodes, while susceptible mice with the same type of pneumococcus show marked changes in the lung and moderate lesions in the spleen and cervical nodes. The method of development of Type III pneumonia, studied by means of serial sections of nasally infected mice, appears to proceed in the stages of vascular engorgement, interalveolar interstitial exudate, albuminous fluid exudate into the alveoli and the perivascular lymphatics draining the affected site, and finally, a frank pneumonia with a cellular exudate in the alveoli but without much fibrin.