[en] In order to use a beam of X-rays (e.g. produced in the form of synchrotron radiation) to pump a Li X-ray laser, it is necessary to remove from the beam (a) all photons which could trigger the laser prematurely, (b) (to avoid excessive heating) most photons lying outside a specified energy band. It is suggested that this can be done by using Li II with properly chosen temperature and density, as a transmision window. The method can be generalized to other laser materials. (Auth.)