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Results of crossed-beam measurements of cross sections differential in ejected electron energy and angle for ionization of atomic hydrogen by 20-1 14-keV protons are reported. Secondary electrons were measured over an energy range of 1.5-300 eV and an angular range of 15°-165°. Atomic-hydrogen targets were produced in a radio-frequency discharge source with a dissociation fraction of about 74%. Ratios of cross sections for H targets to those for Hz targets were obtained from measurements on the mixed target. From these ratios, the measured dissociation fractions, and the absolute cross sections measured for H, targets, the cross sections for H targets were determined. These measurements are compared with the results of the first-order Born approximation, the continuum-distorted-wave eikonal-initial- state approximation, and the classical trajectory Monte Carlo (CTMC) methods. Good overall agreement is found with the CTMC results, except for slow, backward electron emission. The addition of the classically suppressed dipole transitions from the Born approximation to the CTMC results yields a good estimate of the ejected electron spectrum.