The construction and test of a source of polarized electrons is described. The electrons are produced by photoemission with circular polarized Laser light on a GaAs-crystal. The electron beam is accelerated up to an energy of 120 keV which is adapted to the injection energy into the Linac. A spin rotator changes the spin vectors from longitudinal to transversal orientation. The spin rotation is a necessary condition for the further acceleration in the 2.5 GeV-Synchrotron and in ELSA. For the measurement of the polarization degree Mott-scattering on a thin gold target is used. A polarization degree of 30% has been measured. (orig.).