Sixteen patients with Bechterew's syndrome (ankylosing spondylitis, AS) were investigated. All except one of the patients were HLA B27-positive. Peripheral blood lymphocytes from the patients were examined for spontaneous cytotoxicity against 51Cr-labelled K562 target cells. The cytotoxicity index of the non-T lymphocytes was negatively correlated with the serum CRP concentration (Pearson's r = 0.613, p less than 0.02). This finding seems to be caused by an alteration of the natural killer (NK) cell activity of the non-T lymphocytes during the active phase of the disease and not due to serum factors. The activity of the non-T cell fraction was slightly but not significantly reduced in patients with high disease activity (CRP greater than 0.01 g/l) compared with normal controls, (14.6 +/- 2.8 g/l versus 20.4 +/- 2.4 g/l mean +/- SE; p less than 0.10) Serum IgG and IgA concentrations were increased (p less than 0.001 and p less than 0.05 respectively) Interferon caused normal augmentation in the NK cell activity in AS patients, and no correlation was found between serum gamma interferon titres and the NK cell activity.