Ambipolar radial transport (equal ion and electron flux) is not directly measured in tandem mirror experiments because the particle flow does not produce a net electrical current. The first absolute measurements of the ionization source in the Tandem Mirror Experiment Upgrade (TMX-U) plasma have been obtained. These have permitted the determination of the magnitude of ambipolar radial transport from the particle balance. Furthermore, comparisons of the source measurements with a Monte Carlo neutral transport code have shown reasonable agreement. Measurements of the particle balance under several operating conditions are presented. For some of these cases, the ambipolar radial transport is smaller than the other measured losses.