An EPR signal for Mn(III) bound to the metal transport protein transferrin has been detected for the first time. The temperature dependence and simulations of the EPR signal are consistent with the Mn(III) centers being six-coordinate in an elongated tetragonal environment. Thus, the incorporation of Mn(III) within the Tf active site does not vastly alter the coordination number or active site geometry relative to native Fe(III) 2 -Tf. This parallel mode EPR signal for Mn(III) 2 -Tf could prove valuable for future studies aimed at determining the physiological relevance of Mn(III) 2 -Tf.