To evaluate the effects of chondrocytes transplantation on the regeneration of cartilage by intraarticular injection or injection into blood clots at cartilage defects, eight full-thickness cartilage defects were created surgically on the articular surface of each femoral trochlea of two calves. Autologous chondrocytes were isolated individually from the cartilage pieces collected at the creation of defects. And isolated cells were cultured in monolayers for proliferation. Cells were injected into synovial fluid (Group 2, n=11) or into the blood clots at the cartilage defects (Group 3, n=5) of the left femoropatellar joint on weeks 2 and 3, respectively after the operation. The defects (Group 1, n=16) of right femoropatellar joint were left untreated in the control group. After 14 weeks, repaired tissues were evaluated based on gross and histological examinations. In Group 3, more repaired tissues and a better interface between the repaired tissue and host cartilage were observed compared with the results for Groups 1 and 2. Moreover, cartilaginous tissue were observed more in defects of Group 3 than in defects of other groups. In conclusion, the present study suggests that the injection of cells into the blood clot at a cartilage defect might be applicable for the regeneration of damaged cartilage.