An important parameter in the design of mobile radio systems is the co-channel protection ratio. New measurements, based on realistic subjective testing procedures using speech modulation, are presented for the a.m. case. The critical importance of audible carrier interactions is demonstrated.Mean values of protection ratio which were graded by users as ‘perceptible but not disturbing’ decreased from greater than 30 dB to approximately 15 dB when the carrier frequency offset was reduced from 1 kHz to 100 Hz.Extreme variability between subjects makes reliable experimental evaluation of protection ratio an expensive and time-consuming procedure.