A high-intensity subkilovolt x-ray calibration source utilizing proton-induced inner-shell atomic fluorescence of low-Z elements is described. The high photon yields and low bremsstrahlung background associated with this phenomenon are ideally suited to provide intense, nearly monoenergetic x-ray beams. The proton accelerator is a 3 mA, 300 kV Cockroft-Walton using a conventional rf hydrogen ion source. Seven remotely-selectable targets capable of heat dissipation of 5 kW/cm/sup 2/ are used to provide characteristic x-rays with energies between 100 and 1000 eV. Source strengths are of the order of 10/sup 13/ to 10/sup 14/ photons/sec. Methods of reducing spectral contamination due to hydrocarbon build-up on the target are discussed. Typical x-ray spectra (Cu-L, C-K and B-K) are shown.