Tunable, cw, far‐infrared radiation has been generated by nonlinear mixing of radiation from two CO2 lasers in a metal‐insulator‐metal (MIM) diode. The FIR difference‐frequency power radiated from the MIM diode antenna to a calibrated indium antimonide bolometer. Two‐tenths of a microwatt of FIR power was generated by 250 mW from each of the CO2 lasers. The combination of lines from a waveguide CO2 laser, with its larger tuning range, with lines from CO2, N2O, and CO2 isotope lasers promises complete coverage of the entire far‐infrared band from 100 to 5000 GHz (3–200 cm−1) with stepwise‐tunable cw radiation. To demonstrate the usefulness of the technique, we observed the J=4–5 line of CO at 567 GHz.