Articular cartilage is known to have limited ability to heal once injured, and attempts to heal lesions in cartilage have yielded equivocal results. The following experiments were performed to investigate healing in cartilage transplantation of chondrocytes grown in vitro. The knee joint of the New Zealand White rabbit was used as the experimental model. An initial baseline study was made to determine the intrinsic capability of cartilage for healing defects that do not fracture the subchondral plate. A second experiment examined the effects of autologous in vitro grown chondrocytes on the healing rates of these defects. The results were evaluated by qualitative and quantitative light microscopy. In control defects not grafted with chondrocytes, 6 weeks after the initial defect was created, there was little repair. Macroscopic and histological findings were consistent with an osteoarthritic pathology such as synovitis and "cell nests." Macroscopic results from grafted specimens displayed a marked decrease in synovitis and other degenerative changes. Defects which had received transplants had a significant amount of cartilage reconstituted (82%) compared to ungrafted controls (18%). Controls showed a healing rate comparable to that obtained in the initial baseline study.