The lymphocyte response of mononuclear cells (MNC) to Klebsiella (Klebs) was studied in ankylosing spondylitis (ASP) and, for comparison, in patients convalescing from Klebs infection and in HLA-B27+ and B27- healthy blood donors. When large doses of various Klebs cell envelope structures were used, MNC from all healthy blood donors were stimulated to produce LIF but not to synthesize significant amounts of DNA. In contrast, MNC from convalescent patients showed LIF synthesis (but no significant proliferation) even when small doses of the Klebs biostructures were used. This heightened LIF response to Klebs was long-standing and could be demonstrated even after serum antibodies had vanished. Very similarly, MNC from ASP patients did not proliferate significantly, but could be activated to produce LIF even with small doses of Klebs. The heightened LIF response to Klebs in ASP appeared to be correlated with the clinical activity and/or duration of the disease. In contrast, there was no correlation between the presence of HLA-B27 in ASP patients or healthy controls and the LIF or proliferative response to Klebs.