The presence of glial fibrillary acidic protein was tested for in cartilage of bronchi, trachea, pulmonary hamartomas, articular cartilage and chondrosarcomas. The cytoplasm of most chondrocytes in bronchi stained strongly positive, whilst in hamartomas only small foci at the edges reacted. Staining in the trachea was weak. In chondrosarcomas a few cells were positive, but most areas were negative. Articular cartilage was consistently negative. Young chondrocytes expressed the antigen more strongly than mature cartilage. Dedifferentiated cartilage cells such as chondrosarcoma react, in contrast to their cells of origin in articular cartilage.