<div class="htmlview paragraph">The use of a magneto-rheological (MR) fluid brake as a semi-active torsional damper has been investigated. The MR fluid brake can be used as a clutch-type mechanism to transmit torque between a rotating shaft and the brake housing. In that fashion, the MR brake can be used as an adaptive friction damper. This preliminary effort presents the results of experimental testing of an MR brake and its implementation as a variable friction damper on a simple single degree of freedom torsional system. Optimum friction levels are predicted based on established tuning rules. The experimental work verifies the optimality of the tuning for the case of fixed friction dampers and indicates the potential for future application of an MR brake in the control of torsional vibration.</div>