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The Environmental Control and Life Support System (ECLSS) on the International Space Station (ISS) utilizes the Major Constituent Analyzer (MCA) for monitoring major atmospheric constituents from sampling locations within the station. This paper describes the results of the protoflight testing of the first MCA unit. The MCA monitors six major constituents: oxygen, nitrogen, carbon dioxide, hydrogen, water vapor, and methane. In order to monitor these gases the MCA uses a permanent-magnet, single focusing mass spectrometer. This configuration was originally developed for the Skylab metabolic analyzer and most recently was used for the Spacelab's Gas Analyzer Mass Spectrometer, (GAMS). For the MCA application, the mass spectrometer and its support electronics were repackaged to meet the space station requirements.