Measurement of the absolute temperature of microwave noise sources under field conditions is usually achieved at the expense of tedious rf attenuation measurements and the incorporation of cryogenic equipment in the measurement system. Measurement accuracy is determined by the precision to which the effects of passive rf attenuation are known. The technique described does not require this knowledge of the effects of rf attenuation on instrument calibration. The feasibility of this technique has been established by tests performed with a modified Dicke radiometer. The technique establishes the radiometer indicator output zero as a direct reading of absolute zero and provides calibration of the indicator scale directly in degrees Kelvin. Laboratory calibration of an appropriately modified radiometer and measurement of noise temperature near absolute zero are discussed.