1. Articular cartilage from immature rabbits was placed in and near the rabbit knee joints for periods up to ten weeks.

2. Autografts of articular cartilage when placed free in the joint soon became adherent to its synovial lining; the cartilage with its subchondral bone remained viable.

3. Homografts remained viable in the presence of joint fluid, but when in contact with synovium antigenic cellular reaction was produced early. The presence of subchondral bone intensified this reaction and led to graft invasion and destruction.

4. Partial thickness homografts of articular cartilage were also antigenic and were absorbed. When full thickness cartilage was used, this cellular invasion was resisted by the zone of provisional calcification which appeared to function as a physical barrier against antigenic cells of the host.

5. When placed in muscle, both autogenous and homogenous grafts failed to survive through lack of nutrition, although the autogenous subchondral bone remained viable. It is inferred that subchondral circulation is not sufficient for cartilage survival and synovial fluid is essential for its proper nutrition.

6. Surviving immature articular cartilage transplants underwent "ageing" changes.