Our knowledge of the causative factors concerned in the extremely prevalent catarrhal affections of the upper respiratory tract, popularly known as "common colds," is in an exceedingly chaotic state. The epidemiologic evidence seems to indicate clearly that these conditions are infectious; and this has led numerous investigators to attempt to isolate a causative organism. An etiologic relation to these infections has been ascribed toBacillus influenzae, Streptococcus hemolyticus, Streptococcus viridans, the pneumococcus,Micrococcus catarrhalis, Tunnicliff'sB. rhinitisand many other organisms. Some time ago, Kruse1pointed out that, while common colds are undoubtedly infectious, the paucity of bacteria in the secretions and the short time that these few persist militates against the presumptive evidence that they are concerned in the infection. In attempting to trace the causative factor by means other than the bacteriologic methods that had been resorted to previously, Kruse succeeded in producing colds experimentally with a