The authors assessed the plasma viscosity by means of a capillary viscosimeter of their own design and assessed at the same time the concentration of 22 plasma proteins or lipids. Based on the results of these examinations it proved possible to elaborate an original equation which describes the plasma viscosity as a function of the concentration of fibrinogen, alpha-2-globulins (and among them most probably haptoglobin), gammaglobulins, IgA and IgM. The authors discuss in detail the importance of this finding for clinical haemorheology.