We present a model of the thermopower in a mesoscopic tunnel junction between two ferromagnetic metals based upon magnon-assisted tunneling processes. In our model, the thermopower is generated in the course of thermal equilibration between two baths of magnons, mediated by electrons. We predict a particularly large thermopower effect in the case of a junction between two half-metallic ferromagnets with antiparallel polarizations, S(AP)similar to-(k(B)/e), in contrast to S(P)approximate to0 for a parallel configuration. (C) 2002 American Institute of Physics.