The relative percent dose reduction by lead (Pb) of 6-MeV electrons produced by Clinac 1800 for 6 X 6, 10 X 10, 15 X 15, 20 X 20, and 25 X 25 cm2 cones both with and without buildup is measured. The thickness of Pb required to attenuate the intensity of the primary electron beam to 95% and 98% depends upon the cone size and upon the depth in phantom at which transmission measurements are made.