Isothermoremanent magnetization and its relaxation with time have been investigated in ceramics and powders of YBa2Cu3O7-delta at different temperatures after magnetizing the sample in a field of H(ext) < 600 Oe. From the results for powder samples we obtained for intragranular vortices the value of the effective pinning energy U(eff) almost-equal-to 0.7 eV, independent of H(ext). In the ceramic material U(eff) was found to depend strongly on the applied field reaching a value of 5 eV at 10 Oe and decreasing steeply when the magnetizing field was increased. The field dependence is discussed by using a model which takes into account the interaction between vortices.