A high-purity n-type indium antimonide sample mounted in a reentrant cavity and cooled to 4.2"K was operated as a millimeter-and submillimeter-wave detector using a down-conversion process. Power applied at the millimeter-wave frequency CauTeT a Xarige irii"t%e material's conductivity which in turn causes a change in the X-band power absorption, and cavity perturbation techniques were used to analyze the scheme. The detector was operated successfully a t frequencies of 35,60, 80, and 150 GHz with no long-wavelength cutoff frequency observed. A minimum terminalto-terminal conversion loss and NEP of 11.5 dB and 6.8 X 10-11 W per unit bandwidth, respectively, were measured. The response time is limited by the carrier relaxation time which is of the order of lo-' s.