Experimental investigations have been carried out on the reduction of electron contamination of a 6-MV x-ray beam of Clinac model 1800 for square field sizes 5 X 5 to 30 X 30 cm2 in steps of 5 cm and for rectangular field sizes 19 X 7 and 7 X 19 cm2. The electron contamination of both the open beam and the beam with the tray can be effectively reduced by placing a lead foil filter immediately below the blocking tray. Measurements at 100-cm source-skin distance with filter in place showed a reduction in dose in the buildup region and also a displacement of the location of Dmax to greater depths, even for small field sizes such as 10 X 10 cm2.