The authors have confirmed the fact that blood serum and plasma behave rheologically like a true viscous liquid. It is true for whole blood only to a first approximation, but with this reservation they have studied the available data and extended the equation of Bingham and Durham to cover protein solutions of various concentrations and at various temperatures as well as mixtures of proteins and corpuscles present in whole blood. If Φ is the fluidity of whole blood, Φ1 is the fluidity of water and ΔΦ = Φ – Φ1, then ΔΦ = ß1b1 + ß2b2 + ß3b3 + ··· where ß1, ß2, ß3, etc., are constants for the fluidity lowering of the salts, albumin, globulin, fibrinogen, and the corpuscles, etc., present in the whole blood. The conclusions from the data referred to are intended to buttress this simple equation (6).