Tissue localization of collagenous and basement membrane proteins in the extracellular matrix of five sacro-coccygeal chordomas and human fetal notochords was examined immunohistochemically to assess the implications for the histogenesis and histological diagnosis of chordoma. Human fetal notochords and conventional chordomas both exhibited basement membrane proteins (such as type IV collagen and laminin) and type VI collagen on the surfaces of cellular cords. Type II collagen, a main structural protein of cartilage, was also present in both tissues. In the chordomas, however, type II collagen was not so widespread as it was in the notochords, and the predominant collagenous protein was type I. In contrast, an altered deposition of these proteins was noticed in a recurrent tumour which, histologically, showed considerable atypia and eventually metastasized to the liver. The characteristic cartilage-type and basement membrane proteins disappeared and unusual collagen types, such as types III and V, appeared in the stroma. The results further support the notochordal origin of chordoma and suggest that the immunohistochemistry of collagenous and basement membrane proteins may be a helpful criterion for the histological diagnosis and prediction of the biological aggressiveness of chordomas.