The present invention concerns an enhancement of the supercurrent carrying capabilities of bicristalline or polycrystalline high- T c superconductors, i.e. the critical current densities in such superconductors. The current transport properties are improved by chemically altering, especially doping, the superconductors. It seems that a modification of the space-charge layers at the boundary, e.g. by an increase of the mobile charge carrier concentrations particularly in the superconductor's grain boundaries, which concentrations differ from those resulting in optimum superconducting properties of the grains, are responsible for this positive effect.