Inverse muon decay, νμe-→μ-νe, is a reaction whose cross section can be predicted with very small uncertainties. It has a neutrino energy threshold of ≈11  GeV and can be used to constrain the high-energy part of the flux in the NuMI neutrino beam. This reaction is the dominant source of events which only contain high-energy muons nearly parallel to the direction of the neutrino beam. We have isolated a sample of hundreds of such events in neutrino and antineutrino enhanced beams, and have constrained the predicted high-energy flux.