This paper discusses the validation of measurements of O3 distribution in the Earth's atmosphere made by the Millimeter-wave Atmospheric Sounder (MAS), a component of NASA's ATLAS spacelab shuttle package. Measurements of 184 GHz O3 emission from the Earth's limb at tangent altitudes between 20 and 80 km were made with a 50 channel radiometer having a spectral resolution of 200 KHz. In three missions the emissions of O3, H2O, ClO, and O2 were measured between 70 ¿S and 70 ¿N latitudes. O3 mixing ratio calculations and error analysis were performed with the Rodgers optimal estimation method. Data binned in 128 s batches yield mixing ratio profiles with an altitude resolution of 7 km at the O3 peak and an estimated accuracy of 5%. Additional retrievals made with all data from each mission binned in 5¿ latitude zones yield mixing ratios with about 5 km altitude resolution and somewhat improved accuracy. Comparisons with coincident measurements from other space platforms indicate a general agreement within approximately 5%. The data are publicly available through the German Remote Sensing Data Center. ¿ 1998 American Geophysical Union