The 25 MeV electron beam was extracted from a Varian Clinac-35 linear accelerator and made to produce x-rays in thick targets of different materials. The x-ray beams were flattened by filters of various materials. We have found that an aluminum thick target gives a more penetrating beam in the forward direction than does a lead or tungsten target. The x-ray yield in the forward direction from 0-5° is essentially the same for both aluminum and lead targets. At angles larger than 10°, a lead target shows a higher x-ray yield and a more penetrating beam than an aluminum target. The flattening filter material is important. A more penetrating beam is produced if the flattening filter is made of aluminum rather than tungsten or lead. With an aluminum target and an aluminum flattening filter, we obtain the same depth dose distribution from our linear accelerator as we do from our betatron unit operating at the same energy. In the betatron unit, the radiation is produced in a thin target of tungsten and filtered by aluminum. For the situations that arise in radiotherapy we have shown that the beam from an aluminum target/aluminum flattening filter combination can be flattened just as easily as the beam from a lead target/lead flattening filter combination. We conclude, therefore, that contrary to conventional practice, low atomic number materials should be used for the targets and flattening filters of high energy radiotherapy linear accelerators. We plan to extend our investigations of target and filter design to lower electron energies (10-20 MeV). It is hoped that this paper will inspire manufacturers of new linacs to design the head to include aluminum targets and filters so that the optimum characteristics of the beam can be realized.