Isogeneic and allogeneic chondrocytes cultured in collagen gel were transplanted into osteochondral defects in knee joints of inbred strains of rats. At 12 weeks, all eight isografted defects had healed successfully, compared with only four of eight allografted defects (p < 0.05). At 26 and 52 weeks, all defects except one had healed successfully and there was no significant difference in the success rate between the isografted and allografted groups. Control defects that had been implanted with only collagen gel did not heal successfully.