SUMMARYA bacteriological study was made of blood specimens taken repeatedly during a 2-year period from a child with subacute bacterial endocarditis who received intensive treatment with antibiotics. The conventional bacillary form of Corynebacterium sp. was present in the blood and bone marrow of the patient before the beginning of antibiotic therapy and on occasions when the administration of antibiotics was suspended. These were the periods when the patient showed overt symptoms of clinical illness. When antibiotic therapy was adequate to produce clinical remission of symptoms, the infecting organism was not eradicated, but persisted in the blood in a small granule-like form that could be demonstrated and cultured only by highly specialized techniques. The cultural procedures required to bring about reversion of the granule-like form to the conventional bacillary form and the morphology of the various transitional forms that the organism assumed during the reversion process are described and illustrated.