A method of comparing the noise factor of radio receivers is described. This factor is the ratio of the signal/noise ratio in a resistive source at room temperature before connection to the receiver, to that existing in the receiver output. It is assumed that the noise in the source is thermal, i.e. that no extraneous noise pick-up is present.The measurement is made by using a dummy aerial as a source, and raising its temperature until a measurable increase in noise output is obtained. The noise factor can then be deduced from the increase in noise output and the temperature of the dummy aerial.The method has been used at frequencies up to 1 200 Mc/s.The paper includes some practical details of measurement, and experimental results obtained at frequencies of 45, 200 and 600 Mc/s.