To provide a standard temperature noise source for use with S-band radiometers, the noise power from the argon discharge tube CV1881 has been calibrated at 2860 Mc/s using radiometer techniques. The absolute value of the effective temperature of the tube with a discharge current of 180 mA, when mounted across a waveguide parallel to the E-plane and properly matched, is 11 140° K (15.73 dB) with a maximum error of 260° K (0.10 dB), and represents a considerable improvement in accuracy over previous measurements. Since this tube shows a high degree of stability and consistency, it is suggested that it could be used as an absolute standard noise source for the measurement of the noise factor of receivers.