Microphone calibration by the reciprocity technique is performed by national and other high-level acoustic calibration laboratories. In the past, the electrical transfer function of coupled pairs of microphones was determined by manual measurements. As such measurements were very time-consuming, only a few calibrations were performed at a rather limited number of frequencies. A new automated measurement system has been developed. This has significantly increased the efficiency of transfer function measurements and extended the practical applications of the reciprocity technique. The uncertainty of voltage ratio measurements is less than 0.02 dB (k=2) between 40 Hz and 30 kHz, while the repeatability is about two times better. These numbers are so low that they have only a minor influence on the resulting uncertainty of laboratory standard microphone calibrations. Today, pressure response calibrations (according to IEC1094-2) can be performed with an uncertainty of 0.02 to 0.04 dB. The probe for measurement of electrical current through the transmitter and the preamplifier for the receiver microphones are both connected via cables. This makes it possible to place the microphones remotely from the main instrument, as required with environmental testing (pressure, temperature) and free-field calibrations. Calibration results obtained with the new system will be presented.