The elevated serum IgA in seronegative arthritis and psoriasis is not well understood. We examined 112 patients with ankylosing spondylitis for IgA RF, IgG RF and IgM RF by an ELISA method. Two, two and ten patients were RF positive in each Ig class, respectively. Presence of RF was not correlated with Ig concentrations, with presence of HLA-BW, nor with clinical disease signs. It was concluded that RF may be seen in ankylosing spondylitis, but the increased IgA concentrations in such patients are not IgA RE