The source of polarized electrons attached to the 855 MeV race track microtron MAMI in Mainz is based on photoemission from III-V-semiconductors. Strained layers of GaAsP are used at present. Typical degree of spin-polarization of emitted electrons is 75% with 830 mn light irradiating the cathode of the source gun. The light is produced by a gain switched laser diode followed by a laser diode amplifier. The diode is driven by the 2.45 GHz radiofrequency of the accelerator. So a 2.45 GHz pulsed beam in synchronism with the accelerator-rf is injected into MAMI. Thereby a capture efficiency of injection better than 93% is achieved. Beam currents up to 20 muA that are spin-polarized to a degree of 75% are available at MAMI for long term experiments. Total beam time scheduled for experiments is above 2000 h/a.