AbstractThe antigenic phenotype of cartilage and synovial cells from six cases of primary synovial chondromatosis (PSC) was determined. This was compared with profiles similarly obtained for adult and fetal cartilage cells. The Ki‐67 (proliferation‐associated) antigen was present on 40–50 per cent of chondrocytes in the proliferative zone of fetal epiphyseal cartilage but absent in chondrocytes of adult articular cartilage and PSC cartilage nodules. The absence of Ki‐67 antigen suggests that there were no proliferating cells in the synovium or cartilage in these cases of PSC. In PSC alone, some chondrocytes in the cartilage nodules and mononuclear subintimal cells around the nodules also reacted for CD68, suggesting that growth of the cartilage nodules may occur by a metaplastic process. All synoviocytes in PSC were positive for leucocyte common antigen, HLA‐DR, and CD68, a pattern typical of reactive rather than normal synovium.