The expectorant effect of two weeks treatment with cistinexine dihydrochloride (Rec 15/1884/2, Recordati S.p.A, Milano), on mucus transport rate (MTR) was evaluated in a double-blind study on 30 chronic bronchitis patients, 20 treated with active drug (group D), 10 with placebo (group P). After inhalation of nebulized 99mTc labelled human albumin minimicrospheres (HAMM), MTR was determined by external gamma-camera counting over the thorax. To control initial particle deposition a penetration index (PI) was determined as the ratio of activity between peripheral and hilar regions of equal size. Before treatment no differences were found between groups in spirometry, PI, and MTR. After treatment spirometry and PI remained unchanged in both groups. MTR expressed as the half time (T1/2) of the exponential fitting between 0 and 40 minutes showed an interesting behaviour, particularly in group D. Considering the group as a whole, T1/2 did not vary significantly (381 +/- 400 min. before, vs 224 +/- 132 min. after treatment, p = 0.090). However, a statistically significant reduction of the index (815 +/- 396 min. before vs. 258 +/- 120 min. after treatment, p < 0.01) occurred in the seven patients showing at baseline a T1/2 higher than 338 +/- 182 min. (the reference value observed in our laboratory for 8 asymptomatic non smoker subjects). Therefore, the beneficial effect of the drug is seen only in patients with a severe impairment of MTR.