The etiology of B lymphocyte hyperactivity in ankylosing spondylitis (AS) and in reactive synovitis is unknown and data available on the cellular immune system are controversial. We therefore evaluated peripheral blood T cell populations in AS patients and patients with reactive synovitis (RS) using monoclonal antibodies, previously shown to react with all T cells (OKT3), the inducer-helper T cell subset (OKT4) and the suppressor-cytotoxic T cell subset (OKT8). Results were compared with a normal control group and a group of rheumatoid arthritis (RA) patients. In AS an increase of OKT4+ cells was found, but the total number of T helper-inducer cells in peripheral blood was not different from the normal group and the group of patients with RS. Contrary to the results found in RA, where the helper-inducer/suppressor-cytotoxic ratio (OKT4+/OKT8+) was significantly increased, the immunoregulatory ratio in AS and in RS was normal suggesting another mechanism leading to B cell hyperactivity than in RA.