Factors which alter blood viscosity may have important consequences during angiography. The differential effects of various concentrations of five different radiocontrast media on the viscosity characteristics of erythrocyte-plasma suspensions were made over a range of applied shear rates. The results showed that, at both high and low shear rates, the rate of change of viscosity with contrast concentration differs markedly between the various types of contrast media. The conventional ionic monomers caused most disturbance to blood viscosity. The monoionic dimer hexabrix was least disturbing to the viscometric characteristics of blood, and the newer non-ionic monomers were intermediate in their effects. Significant effects on blood viscosity may be caused by radiocontrast agents during a number of in vivo angiographic situations, in particular: early after contrast bolus injection into large vessels, in the microcirculation after selective injections, and during angioplasty procedures.