Neutrino mass constraints on mu decay and pi0--> nunu.

In this Letter, we show that upper limits on the neutrino mass translate into upper limits on the class of neutrino-matter interactions that can generate loop corrections to the neutrino mass matrix. We apply our results to mu and pi decays and derive model-independent limits on six of the ten parameters used to parametrize contributions to mu decay that do not belong to the standard model. These upper limits provide improved constraints on the five Michel parameters, rho, xi', xi", alpha, alpha', that exceed Particle Data Group constraints by at least one order of magnitude. For pi(0) --> nunu we find, for the branching ratio, B(pi(0) --> nunu) < 10(-10).