The occurrence, distribution and nature of cell death has been studied in the interzone of the developing knee joint of the chick by optical, transmission and scanning electron microscopy. Cell death precedes and accompanies the process of formation of the joint cavity (apart from the femoropatellar component), and is also associated with the surfaces of intra-articular structures as they differentiate within the interzone. Cell death in the knee joint shows many of the morphological characteristics of apoptosis. It is proposed that cell death is one factor in the process of cavity formation in the joint.