Dynamometer tests of a production disc brake provided new information on asbestos fiber emissions during break-in, normal use, and high temperature use conditions. Both ambient air and brake cooling air were sampled isokinetically, using 0.45 μm filters. Examination of test and background filters required a clarification process to maximize fiber detectability, the use of transmission electron microscopy (at 40,000X) for detection, and electron diffraction for positive identification of asbestos fibers. Most of the lining asbestos was found to be converted to a nonfibrous material by the high flash temperatures of the braking surface. Less than 0.02% of the lining wear was released as asbestos fibers. The concentration of asbestos fibers in the urban atmosphere, due to brake usage, was conservatively estimated at less than 0.07 X 10-9 g/m3. Based on this upper bound, the use of brakes was judged to be not significant as a source of atmospheric asbestos.