During December 1993 a microwave radiometer belonging to the Central Aerological Observatory (CAO) Moscow was operated at the Meteorological Research Unit, Cardington, UK. The radiometer has a single channel with a central frequency of 61.0 GHz, bandwidth of 2 GHz and sensitivity 0.04 K for a 1 s integration time. The antenna (beam-width 6°) was scanned from nadir to horizontal in 9° steps. Calibration was done every scan cycle using an electrically switched load. The information potentially available from such a radiometer is discussed. The measurements made at Cardington are compared with in situ observations made by radiosondes and by the Meteorological Office's tethered balloon system. Microwave brightness temperatures calculated from the radiosonde ascents are compared with observations. These were found to agree with a mean difference of 0.1 K and with a standard deviation of < 0.3 K. Similarly retrieved temperature profiles up to 500 m were found to agree with the radiosonde profiles within 1 K rms. During the comparison period there were no strong low-level inversions to test fully the radiometer's retrieval capability.