The levels of proteoglycan antigen were measured in joint aspirates from the hip of children with transient synovitis, septic arthritis, Legg-Calvé-Perthes' disease and congenital and traumatic dislocation. Significantly increased levels were found in children with transient synovitis and septic arthritis as compared with other conditions. We propose that the proteoglycan antigens in the joint fluid were released from the articular cartilage in a partially degraded form as a result of an increased rate of proteolytic degradation. In transient synovitis, the source of proteolytic activity may be chondrocytes activated by factors released by synovial cells. The release of joint proteoglycan may cause a temporary increase in deformability of the hip cartilage of the child that could be an important pathogenetic mechanism in some of the sequelae of these diseases.