Acute rheumatic fever and rheumatoid (infectious) arthritis are often indistinguishable clinically,1particularly at the onset. They may both start acutely with an infection of the upper respiratory tract or with epistaxis. A moderately high temperature, chilliness, perspiration, prostration, spreading and transitory involvement of joints, acutely inflamed joints, leukocytosis and anemia are seen in the two conditions. Furthermore, the spleen and lymph nodes may be enlarged in each. Even the subcutaneous nodule,2hitherto considered to be one of the most characteristic phenomena of acute rheumatic fever, is definitely observed in rheumatoid (infectious) arthritis and its incidence3reported with increasing frequency by those who search for it. The sedimentation rate, too, no longer serves as a distinction, for the two conditions have been found to present increased rates of sedimentation of the red blood cells.4 Although in recent years numerous pathologic, bacteriologic and immunologic investigations5have increased