Subpopulations of human blood T lymphocytes were determined in 20 patients with ankylosing spondylitis and in 20 healthy individuals. The proportion of T lymphocytes with receptors for the Fc portion of IgG (TG cells) was increased in 9 patients. The proportion of T lymphocytes bearing receptors for the fc portion of IgM (TM cells) was within the range found in healthy controls in all but 4 patients, who had a decreased level of TM cells.