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.In order to comply with maintainability requirements, the MCA flight design was divided into seven Orbital Replaceable Units (ORUs):ORU 01 Data and Control Assembly, ORU 02 Major Constituent Analyzer Assembly, ORU 04 Low Voltage Power Supply Assembly, ORU 05 Series Sample Pump Assembly, ORU 06 Sample Distribution Assembly, ORU 07 EMI Filter Assembly, and ORU 08 Verification Gas Assembly. All ORUs are mounted on the chassis assembly, AMI 02.Protoflight testing consisted of a sequence of qualification tests performed on the first system to be used for flight. The test levels were reduced from qualification levels in order to minimize stress on the unit.The testing performed on the MCA consisted of an initial performance test, software validation tests, environmental test and a final performance test. The initial performance test demonstrated that the MCA met all of its requirements. Then the environmental qualification tests were run at protoflight levels.- The environmental tests performed were EMI, Acoustic, Thermal, Humidity, Depress/Repress, 1553 Remote Terminal Validation and Waveform Testing, and Vibration. The final performance test ensured that none of the previous tests degraded the performance of the MCA. The test results show that the MCA met all of its performance requirements and environmental requirements. There were several lessons learned from the test such as carbon dioxide accuracy requirements and maintenance of the MCA.