For neutrinos with a magnetic moment, we calculate the production rate of right handed neutrinos in a hot and dense plasma via the quirality flip(&ngr;L→&ggr;*&ngr;R)and the plasmon decay(&ggr;*→&ngr;L&ngr;R)processes. The rate for these processes is computed in terms of a resummed photon propagator which consistently incorporates the background effects. Applying the results to the case of supernova collapse, our results can be used to place an upper limit on the neutrino magnetic moment&mgr;&ngr;<(0.1–0.4)×10−11&mgr;B.©1999 American Institute of Physics.