Thirty-four patients, 21 male and 13 female, with chronic asthma and tenacious mucoid expectoration were studied regarding clinical parameters, PEF, airway resistance and sputum viscosity measured according to the n.m.r. method (nuclear magnetic resonance); a technique for determination of the water phase viscosity. Using double-blind cross-over technique, oral treatment with the two drugs and placebo was given during three consecutive 12-day periods with 48-h intervals free of test drug. No significant improvement in clinical state, PEF or airway resistance could be demonstrated. Nor did the sputum viscosity show any significant change after treatment with the different preparations. However, regarding the patients' own preference of drug as to mucolytic effect, a significant difference in favour of bromhexine at the 0.1% level was shown.