2 h after the inhalation of monodispersed 99mTc-labeled autologous spherocytes and of commercial human albumin microspheres (HAM), 7 patients with chronic bronchitis underwent bronchofibroscopy. The fate of organic particles along the tracheobronchial tree was verified by scanning electron microscopy and the proteolytic activity (trypsin and PZ peptidase) in mucus samples was assessed. Significant proteolytic activity was detected in bronchial secretions. Thereafter in vitro digestion of labeled spherocytes and HAM was verified after exposure to increasing concentrations of trypsin. While in vitro a similar time-course of tryptic digestion of both particles was observed, in vivo spherocytes seem to be less vulnerable to enzymatic digestion. These findings add another unexpected variable, which may influence the reproducibility of radioaerosol lung mucociliary clearance measurements, and improve its standardization.