Cartilage formed in transplants of allogeneic chondrocytes into joint cartilage defects in rats was infiltrated by immune cells migrating from the bone marrow while the surface on the side of the joint cavity remained free of infiltrations. This suggested that immunization occurred via bone marrow and not via joint cavity. Because articular cartilage is nourished exclusively by the synovial fluid, we have attempted to prevent cartilage rejection by protecting transplants from the contact with bone marrow. Defects in articular surface were filled with bone cement and chondrocytes were transplanted into a cavity prepared within the bone cement plug. Cartilage formed within the cement shell remained free of infiltrations and did not evoke systemic immunological response. However, distribution of glycosaminoglycans in the matrix of protected transplants was irregular. Cultures of chondrocytes growing in vitro on cement contained less glycosaminoglycans than the controls. This suggests that some factor(s) released from the cement unfavorably influenced chondrocytes and matrix production in protected transplants.