In 1882 Friedländer1 described a gram-negative, encapsulated bacillus which was at first regarded as the cause of pneumonia. Four years later Weichselbaum2 demonstrated that Diplococcus pneumoniae was the more frequent etiologic agent. Recent extensive studies show that Bacillus Friedländer causes about 0.5 to 4.0 per cent of all the pneumonias and accordingly will be encountered several times a year in any large medical service.
In 1925 Avery, Heidelberger and Goebel3 demonstrated the chemical and immunologic relationship of certain strains of this bacillus to Pneumococcus type II. The following year Julianelle4 classified a number of strains of B. Friedländer by immunologic methods under types A, B and C with a heterologous group X. There are now available the Friedländer bacillus A and the Friedländer bacillus B serum for identification of strains by the Neufeld technic, but the B strain also swells with the type II pneumococcus typing