A method for accurate measurement of beats on ciliated respiratory epithelial cells in vitro is described. The technique allows the frequency of ciliary beats to be recorded directly on minute specimens of human respiratory epithelium obtained by bronchoscopic brushing. A "hanging-drop" preparation of the scraped material is placed on a thin, flat welled slide and is viewed with a phase-contrast microscope at a magnification of x450. Light transmitted through the specimen is deflected in varying intensities due to the beating action of the cilia and is detected by a photomultiplier cell, amplified and transduced as peaks on an oscilloscope. Due to the relatively high magnification, a small group of cilia may be focused upon and their activity measured. Specimens from two groups totaling 53 patients were measured; in the first group the frequency of ciliary beats was measured at 23 degrees C and ranged from 9.1 to 12.9 beats per second with a mean and standard deviation of 11.0 +/- 1.3 beats per second. The second group was measured at 37 degrees C, and values ranged from 10.3 to 16.8 beats per second, with a mean and standard deviation of 13.8 +/- 1.8 beats per second. These values are interpreted to reflect the autonomous frequency of ciliary beats of the isolated respiratory cells. This simple method may be applied to screen for abnormalities of ciliary beating in patients with altered respiratory mucous clearance, as well as to examine the specific effects in vitro of chemicals, drugs, or pollutants on human ciliary activity.