During the past year, a study has been made of the sedimentation rate of the red blood cells in children, with a view of obtaining information which might be of practical clinical value. A study of the literature on the sedimentation reaction reveals a comparative scarcity of reports concerning pediatrics;1 the majority of communications pertain to gynecology and obstetrics 2 and to tuberculosis.3 The history of the development of the sedimentation reaction begins with observations made by Galen, Hewson, Hunter,4 Biffi5 and others. In 1897, Biernacki 6 proposed a method for determining the sedimentation reaction and reported clinical and experimental observations. Fahraeus 7 proposed another clinical method in 1918. Westergren 8 modified the procedure in 1920 and subsequently, Linzenmeier proposed a simplified procedure 9 and has added a micromethod.10 Many other modifications of the technic and new methods have been offered,11 all based on the same principle, namely, the time required for the erythrocytes to settle out of their plasma suspension. A number of theories have been proposed in explanation of the sedimentation reaction,12 and considerable experimental work has been done.13 Fahraeus 14 stated that the change in sedimentation is a physicochemical phenomenon dependent on a varia¬ tion in the properties of the plasma; that is, a qualitative change in the serum globulin, principally fibrinogen. Since the globulin has a high agglutinative power and since the agglutinative capacity of protein is related to the colloidal state, it follows that the suspension stability of the cells is dependent on the colloidal state. The majority of investigators agree on this premise.15 The fibrinogen content of the blood is increased in certain diseases 15a-leukemia, pneumonia, pleurisy, rheumatic fever, erysipelas, suppurative infections, diphtheria, scarlatina, rubeola, typhoid fever, influenza, tuberculosis and syphilis. In these diseases, the rate is usually most rapid. The fibrinogen content of the blood is decreased