Cross sections have been calculated for single ionization in collisions between protons and helium atoms, in the range of projectile energies 25-100 keV. The semiclassical impact parameter method was employed, with a basis comprising 22 states on the (He) target and 29 states on the projectile. Good agreement is obtained with the measurements of Shah et al. (1989). The inclusion of pseudo-continuum states on the helium centre is found to significantly modify the total cross section for capture into the bound states of hydrogen, leading to improved agreement with the measurements.