The macromolecular proteins (greater than 100,000 daltons) have proved mainly responsible for the protective power of mucus against penetration of the H+ ion into the surrounding tissues. This fraction is also mainly responsible for the buffer capacity and, owing to its content of glycoproteins, for the pH-dependent viscosity of mucus. Viscosity determinations, both on native sputum and on reconstituted human alpha-acid glycoprotein, indicate that either raising or reducing the pH from neutral results in an increase in viscosity. The results suggest that individuals with low pH and/or a low protein concentration (less than 6 mg ml-1) or low buffer capacity (less than 3 mumols H+/pH unit) in their mucus, i.e. a low protective power of their mucus, will risk effects released from the underlying tissues when exposed to acidic pollutants. Alternatively, persons with higher concentrations of mucus proteins will risk effects caused by increased mucus viscosity. Sputum was tested from six smokers without any symptoms other than the ability to clear their throats easily.