From the time of the early Greek physicians many observers noted a relationship between the sedimentation rate of the red cells and the concentration of fibrinogen or " phlegma " in the blood. However, no detailed studies were made until 1918 when Fahraeus (5) studied the sedimentation rate in pregnancy and concluded that the increase in the rate was due to a lowering of the electric charge on the red cells. Since that time interest in the subject has been renewed. Numerous investigators have studied the variations in sedimentation rate that occur in disease and a few workers have attempted to determine also the factors underlying these variations. As a result of these investigations of the past twenty years, the sedimentation rate has become of definite clinical value, but there is no general agreement as to the factors involved in the aggregation and sedimentation of the red cells.The majority of investigators have corroborated the long-standing impression that there is a suggestive relationship between the concentration of fibrinogen and the sedimentation rate. In fact, despite occasional marked exceptions to an exact linear relationship between the fibrinogen and the sedimentation rate, the majority of recent workers have concluded that the concentration of fibrinogen determines the sedimentation rate (1,2,4,8,10,14,16). The occurrence of occasional marked inconsistencies, however, has led a few workers to conclude that the relationship between fibrinogen and sedimentation rate is not one of cause and effect (9,12,13 that blood samples with high concentrations of globulin have high sedimentation rates.In the course of our studies of the sedimentation rates and plasma protein fractions in arthritis, we occasionally obtained marked inconsistencies in the relationship between plasma proteins and sedimentation rates, in contrast with the majority of the findings reported in the literature. Because of these findings and the lack of agreement as to the correlation between the plasma proteins and the sedimentation rate, which is evident from a survey of the literature, we undertook a more detailed study of the relationship.The present investigation includes chiefly studies in various types of arthritis, but it also includes a few studies in other diseases which produce abnormal rates, such as myelomatosis, carcinoma of lung, malignant lymphoma of Hodgkin's type, acute lupus erythematosus disseminatus, poikiloderma atrophicans vasculare, lymphogranuloma inguinale, and nutritional edema. The results are of special significance since they cover both the sudden and marked changes in sedimentation rates and in concentration of proteins that occur in acute infections such as gonorrheal arthritis, and the more gradual changes in chronic infections such as rheumatoid arthritis. In conditions in which the type of reaction is so different, the chemical changes produced by the infection would be expected to be different. Whatever the stimulating agent may be which is produced by the infection and which leads to changes in the...