In this study we compared cytokine production and cell proliferation of immunocompetent cells derived from patients with ankylosing spondylitis (AS) to those from healthy blood donors using a whole blood assay. To this end, blood cell cultures were stimulated with the superantigens MAS (Mycoplasma arthritidis supernatant) and staphylococcal enterotoxin B (SEB) and the plant lectins phytohaemagglutinin (PHA) and concanavalin A (Con A). The number of white blood cells (WBC) and lymphocyte subsets were also determined. Cell proliferation and levels of interferon-gamma (IFN-gamma), interleukin-1 beta (IL-1 beta) and interleukin-6 (IL-6) were measured after stimulation with the different mitogens. An ELISA test was used to analyse supernatant cytokine levels. Individuals with AS showed significantly lower IFN-gamma concentrations and markedly lower cell proliferation rates with all tested mitogens than healthy controls, while there was no significant difference in IL-6 synthesis. IL-1 beta levels were slightly impaired in the patient group, but only blood cell cultures stimulates with MAS showed a statistical significance. Furthermore, there was a significant elevation of leucocytes and lymphocytes in patients with AS resulting in higher numbers of CD4-positive cells, which implies a higher CD4:CD8 cell ratio. CD19- and CD8-positive cells were not significantly distinct compared to healthy controls. This deviation in cytokine levels and cell proliferation points to a suppression of T lymphocytes. A disturbed T-lymphocyte function may play a part in the pathogenesis of AS.