The injection of an electron beam into a weakly collisional magnetized background plasma is investigated experimentally. The beam‐plasma system is so large that boundary effects are unimportant for establishing the current flow. It is shown that the injected beam current is balanced by a field‐aligned return current provided by background electrons drifting toward the beam source. The conditions for an ion sound instability driven by the return current are met. The maximum injected beam current is found to be limited to the background electron saturation current to the beam source.