Human nasal and bronchial epithelial cells were cultured in vitro and compared morphologically and functionally. Morphologic assessment by both light and electron microscope and indirect immunoperoxidase staining techniques confirmed the identity of the two cell types as being epithelial. Light microscopy of confluent cultures revealed tightly packed cell monolayers, whilst electron microscopy showed that cells were linked by tight junctions. Estimation of cell size by planimetry found these cells to have a mean width of 10.6 +/- 1.1 microns for nasal cells and a mean width of 10.2 +/- 1.0 microns for bronchial cells. A high proportion of both the nasal and the bronchial cells exhibited features of the mature ciliated cell types, and constituted between 50 and 76% of the total cells at the earlier stages of culture although this decreased to between 16 and 23% of the total by 4 weeks in culture. The ciliary beat frequencies of the nasal and bronchial cells were found to be similar at 10.8 +/- 0.7 Hz and 11.8 +/- 2.3 Hz, respectively. The cilial beat on adjacent cells was synchronous, suggesting the presence of intercellular communication between the neighbouring cells. These studies demonstrated that there was little difference between the cultured nasal and bronchial epithelial cells with respect to either their morphology or ciliary activity.