Elastic scattering of 100 MeV protons by 3He and 4He has been measured over a wide range of angles. Analysis of the small-angle results was carried out using a superposition of nucleon-nucleon amplitudes computed with both modern and old phase shifts. No significant differences were found in the accuracy of the predictions resulting from the two sets, but in both cases the 3He results were better than those of 4He. The peaking evident in the backward p–4He scattering but not seen in heavy nuclei has been interpreted in terms of an exchange interaction using an optical model analysis.