Investigations of high-energy plasma require probing techniques that do not change the plasma characteristics. A molecular beam is such a probe for plasmas that have energetic electrons and cold ions. Attenuation of the beam is mainly by electron ionization, but other single-particle processes must be considered. A beam of hydrogen molecules is here used as the probe. That portion of the neutral beam that passes through the plasma is measured as a pressure by a specially designed differential manometer. The measured attenuation is then used to calculate plasma density using known values of the cross sections for the interactions between beam molecules and plasma electrons and ions.