Previously, we perfused the lungs of several mammals (rat, mouse, guinea pig and hamster) with the detergent-containing medium and found that abundant filaments were present in apical caps of nonciliated bronchiolar epithelial (Clara) cells. These filaments were bound to the plasma membrane of the apical cap at one end and ran into the interior of the cytoplasm at the other end. In the present study, we used heavy meromyosin (HMM) and an immunohistochemical method to identify these filaments in rat Clara cell. Most filaments in apical caps bound HMM, but some filaments were not decorated with HMM. To examine the latter filaments, paraffin sections were stained immuno-histochemically utilizing the antibodies against intermediate filament proteins. Apical caps were positive for cytokeratin but were negative for other intermediate filament proteins, vimentin, desmin, neurofilament protein and glial fibrillary acidic protein. These results suggested that filaments in apical cap of nonciliated bronchiolar epithelial (Clara) cells were composed of actin and cytokeratin.