The new method presented is suitable for rapid testing of i.f. and microwave amplifiers under operational conditions. The input to the amplifier under test is a carrier simultaneously modulated in amplitude and phase, and the output is displayed on an oscilloscope by a spectrum analyser having special facilities for comparing the two sidebands and rejecting the carrier frequency. The technique of measurement is based on compensating the unknown phase modulation of the output signal with the known input phase modulation, the exact equality being indicated by noting the coincidence and minimum value of the two sidebands. The a.m.-to-p.h.m. conversion is then read on two instruments calibrated directly in decibels and degrees, and the a.m. compression is determined by reading the decibel scale of a u.h.f. attenuator.The new method is compared with those already known from the literature, and it is stated that the accuracy of measurement is primarily determined by the accuracy of measuring attenuation. A short analysis, upon which the new method is based, is given, and data for the equipment actually built are included, together with some typical oscillograms.