We develop a theoretical model of magnon-assisted transport in a mesoscopic tunnel junction between a ferromagnetic metal and a normal (nonmagnetic) metal. The current response to a bias voltage is dominated by the contribution of elastic processes rather than magnon-assisted processes and the degree of spin polarization of the current, parameterized by a function P(Pi(up arrow(down arrow)), Pi(N)), 0less than or equal toPless than or equal to1, depends on the relative sizes of the majority Pi(up arrow) and minority Pi(down arrow) band Fermi surface in the ferromagnet and of the Fermi surface of the normal metal Pi(N). On the other hand, magnon-assisted tunneling gives the dominant contribution to the current response to a temperature difference across the junction. The resulting thermopower is large, Ssimilar to -(k(B)/e)(k(B)T/omega(D))P-3/2(Pi(up arrow(down arrow)), Pi(N)), where the temperature dependent factor (k(B)T/omega(D))(3/2) reflects the fractional change in the net magnetization of the ferromagnet due to thermal magnons at temperature T (Bloch's T-3/2 law) and omega(D) is the magnon Debye energy. (C) 2003 Elsevier B.V. All rights reserved.