Macrolide antibiotics have a variety of actions along with antimicrobial action. To determine the effects of oral administration of clarithromycin (CAM) on rheological properties, we measured the spinability, dynamic viscoelasticity, and solid composition of human nasal mucus from 18 patients with chronic sinusitis before and after administration of CAM for 4 weeks. After administration of CAM, the spinability and percent solid composition of nasal mucus increased from 26.5 +/- 12.2 mm to 40.2 +/- 18.7 mm and 7.86% +/- 3.47% to 13.90% +/- 3.67% (p < .05), respectively, whereas the ratio of the viscosity to the elasticity (eta'/G') of nasal mucus after the administration of CAM decreased in all of the mucus samples. These results suggest that treatment with CAM may modulate the rheological properties of nasal mucus in patients with chronic sinusitis.