Three T-cell lines and clones of the OKT4 phenotype have been isolated from the peripheral blood of three patients with ankylosing spondylitis. Antigen specificities of T cells were determined with purified protein derivative-(PPD) and cartilage-derived antigens, namely proteoglycans from human articular cartilage and intervertebral disc, bovine nasal cartilage, and rat chondrosarcoma and human type II collagen from cartilage. A cell line from one patient reacted with proteoglycans from human articular cartilage and human intervertebral disc, but the other two cell lines (each from a different patient) and four clones from one of the latter two lines proved to be highly specific for the human articular cartilage proteoglycan. From a study of four proteoglycan specific clones isolated from one patient, it is clear that removal of chondroitin sulfate had no effect on immunoreactivity but digestion of proteoglycan with pronase or alkali/sodium borohydride treatment abolished all reactivity. A OKT4-positive T-cell clone isolated from a healthy adult which was reactive to PPD was used to compare the antigen specificity of cells: this clone showed no reactivity to any of the other putative antigens listed above.