An algorithm using NOAA-12 AVHRR (Advanced Very High Resolution Radiometer) solar reflectance measurements for retrieving cloud droplet size and optical thickness has been applied to a boundary layer stratocumulus cloud in the vicinity of the Azores on 12 June 1992 during the Atlantic Stratocumulus Transition Experiment (ASTEX). This day was particularly advantageous for validations because of the absence of cirrus or other higher-level clouds during the satellite overpass and the existence of a large relatively uniform stratus cloud deck. Uncertainty estimates for the retrievals are presented along with a discussion of the algorithm. An in-flight absolute calibration of AVHRR channel 1, necessary for accurate optical thickness retrievals, was done concurrently with the retrievals through comparison with a visible radiometer flown on the National Aeronautics and Space Administration's ER-2 and using the stratus cloud as the common reflectance target. Results are compared with in situ microphysical measurements taken with the Particulate Volume Monitor (PVM-100) and Forward Scattering Spectrometer Probe (FSSP-100) instruments on the University of Washington C-131A aircraft. Satellite retrievals of both optical thickness and droplet size lie within the values measured by the two in situ instruments.