The 5-Fluorocytosine bronchial secretion levels of 14 patients with a chronic respiratory disease were determined using a microbiological method. The mean maximal concentrations (7.76 +/- 7 micrograms/ml) were obtained at the end of a single 25 mg/kg i v perfusion over 30 minutes. The level increased when five i v perfusions were performed every 6 hours and bronchial diffusion was higher and faster by concomitant administration of (bromhexin hydrochloride). Therefore, 5-Fluorocytosine was not found in bronchial secretions of four patients in spite of a normal serum levels (21.5 +/- 5 micrograms/ml).