Resistive measurements of the critical current densityJcin transverse magnetic fields were made on single crystals of niobium, which had been subjected to systematic thermal, mechanical, and chemical treatments in order to produce a wide range of substructural densities, and variations in the oxygen content. The introduction of plastic deformation into niobium crystals produced a peak inJcat applied magnetic fields of strengthHc2, the upper critical field. The peaks inJcwere removed by annealing the crystals in the recovery range. The critical current density was increased both below and aboveHc2by producing a dislocation substructure in the crystals. Small additions of oxygen also causedJcto increase above and belowHc2. The upper critical field, however, was relatively insensitive to structure, varying only through the structure sensitivity of the bulk GLAG parameters.