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We have built and operated a system which monitors the gains of 318 lead-glass counters as a function of time with an accuracy of 1-2%.The light source is a single high-intensity yellow light-emitting diode (LED), and the light is transmitted to each of the counters by low-attenuation plastic o~tical fibers. The intensity of the LEO light is measured by a reference scintillation counter whose gain is known from

A "Lead-Glass Wall," consisting of 31B lead-glaaf Chercnkov shower counters and three wire spark cham bers, has been added to one octant of the SPEAR Mark I Magnetic Detector. The wall covers a solid angle of approximately 62 of 4TI steradians and has been used to Identify and measure the energies of elec trons and photons produced 1->leetron-posirron col lisions.The design, calibration, gain-monitoring, and performance of the system are described.