Curschmann's spirals from 30 patients suffering from chronic bronchitis were examined histochemically by periodic acid-Schiff (PAS), Alcian blue (AB), PAS-AB, high-iron diamine (HID), and HID-AB stains. Their central core and fibrillar component were found to be rich in neutral mucins and in sulfated and nonsulfated acid mucins, while their outer mantle was rich in neutral and nonsulfated acid mucins only. Curschmann's spirals consist of mucus of high viscosity. Histochemical and morphological evidence suggests that, in addition to their origin in the lumen of smaller bronchi and bronchioles, they are also formed in the ducts of the seromucous bronchial glands.