Abstract Effects of partial substitution for constituents of J c and U o were studied in Bi 2Sr 2Ca 1Cu 2O x single crystals. Pb, Ba and Li substitution was effective in slightly improving T c , but Y degraded it significantly. Although U o was insensitive to or showed a slight increase with Ba and Li substitution, the value of J c determined by the magnetic relaxation tended to decrease. Y substitution caused a considerable degradation of both J c and U o . On the contrary, Pb substitution for Bi at about 17% gave a remarkable enhancement of J c to about 10 6 A/cm 2 at 4.4 K, while U o was rather insensitive to the substitution.