We studied production of tumour necrosis factor (TNF) and interleukin 1 (IL-1) by using purified peripheral blood monocytes of patients with previous yersinia arthritis (YA) and of healthy HLA-B27 positive and negative controls. Lipopolysaccharide-exposed cells of HLA-B27 positive and negative patients, and those of HLA-B27 positive controls, generated significantly more TNF than did HLA-B27 negative control cells. There was a positive correlation between the levels of TNF and IL-1. Our results give credence to the view that augmented production of phlogistic mediators may contribute to inflammatory symptoms in patients with HLA-B27 associated disease.