In 1897 Bordet1 drew attention to the fact that there generally is more or less hemolysis in the blood of a rabbit which has died of streptococcus infection. This hemolysis is not constantly observed, but was present, for instance, in 37 of 40 rabbits dying of streptococcus infection, in which it was looked for in this laboratory during the last eight months. This phenomenon seems to be unique in the field of bacteriology, although there are several other organisms, such as the B. tetanus,2B. pyocyaneus,3Staphylococcus pyogenes,4B. typhosus,5M. tetragonus,6B. megatherium,7 and others,8 the filtered cultures of which are capable of hemolysing red corpuscles of various animals. These organisms, however, do not produce hemolysis in the body of an animal which has been injected with the young cultures, as is the case with the streptococcus. Todd7 has found, however, that filtrates from an old culture of B.