An investigation was made of the excitation and amplification of 100-1000 MHz elastic waves in lead molybdate crystals by radiation emitted from a Q-switched ruby laser. The elastic waves were excited by mixing, in a lead molybdate crystal, two laser beams shifted in respect of the frequency and intersecting at twice the Bragg angle. Interaction of the laser beam with externally injected ultrasonic waves amplified these waves by up to 10 dB. The results are theoretically analyzed.