The viscosity of isolated human salivary mucins has been studied as a function of shear rate, mucin concentration, pH, and ionic strength. At neutral pH, viscosity increased proportionally with mucin concentrations between 0 and 14 mg/ml. Increasing the ionic strength from 35 to 235 mM resulted in an approximately 50% decrease in specific viscosity. A part from the ionic strength effect, no specific effect of calcium ions was observed. Under low ionic strength conditions, viscosity of mucin solutions reached a maximum value at pH 4.2. The extent of the viscosity increase was dependent on ionic strength, mucin concentration and shear rate. Increase of the ionic strength up to 200 mM almost completely abolished the pH-optimum, suggesting that electrostatic interactions underlie the pH-dependent behaviour. The position of the pH-optimum was not changed upon desialization of the mucin, indicating that terminal sialic acid residues do not determine the pH-dependence of human salivary mucin viscosity.