It was the purpose of this study to search for distinctive features in the group of streptococci associated with subacute streptococcus endocarditis. The criteria adopted were cultural, biochemical and immunologic characteristics. Incidentally, the presence of antibodies in the blood of patients suffering from the disease and the immediate effect of blood transfusions on the bacteriemia were investigated. The recognition of a distinct form of endocarditis, called by some writers subacute bacterial, by others endocarditis lenta and chronic infectious endocarditis, has been of comparatively recent date and of slow evolution. Although of only moderately common occurrence, the disease is extremely important, since all of the cases studied, except one,1 have been fatal. Virchow in 1855 first suggested an infectious origin for the endocarditis associated with uterine infections, and attention was thereafter directed to the finding of bacteria in the vegetations in both ulcerative and verrucous endocarditis. Heiberg2