Waaler1and Rose and co-workers2made the puzzling observation that the sera of patients with rheumatoid arthritis agglutinated sheep red cells sensitized by subagglutinating doses of rabbit anti-sheep hemolysin. Although this mystery has not been completely elucidated as yet, great progress has been made. The rheumatoid factor (for reference see Ziff,3Vaughan,4and Franklin and collaborators5) has been found to react with gamma globulins of various species, including man. This has been demonstrated by various serological procedures, including agglutination of sensitized red blood cells, agglutination of tanned and coated erythrocytes, latex fixation, and precipitation. According to Franklin and colleagues,6the rheumatoid factor is a gamma globulin of high molecular weight. A somewhat similar factor was described in the sera of patients with various diseases, as well as those of apparently normal human beings.7This factor reacted only with red blood cells sensitized by