TOMS has been the main satellite instrument for measuring the global distribution of the total atmospheric column of ozone since the first one was launched in 1978. The fifth instrument's launch is planned for August 2000. A key scientific objective of the TOMS mission is to monitor the trend of total global ozone, which requires the ability to detect a 1% change in ozone over a decade. This, in turn, requires high calibration accuracy and long-term stability in the TOMS ratio measurements between the solar spectral irradiance and the Earth spectral radiance. The calibration process requires not only knowledge of the radiometric response of the instrument, but also of various instrument characteristics to convert the instrument output to the value of the physical observable being measured. This is due to the fact that the object sources in measurements may have different characteristics from those of the radiometric standards, e.g., intensity, polarization, and spectral distribution; the process of calibration requires a complete set of instrument characteristics, e.g., linearity, spectral bandwidth, and straylight response, to compensate for the difference between the standards and the source being measured. This paper describes methodologies of the TOMS FM-5 prelaunch tests that are relevant to calibration.