In 75 patients with unexplained chronic purulent rhinosinusitis T cell mediated immunity to three micro-organisms frequently colonizing the human upper respiratory tract, viz. Haemophilus influenzae, streptococci and Candida albicans, was assessed. Delayed type hypersensitivity (DTH) skin test reactivity was measured in vivo, whereas the blastogenic responsiveness (lymphocyte transformation test; LTT) and lymphokine production (e.g. migration inhibition factor; MIF) of the lymphocytes upon antigen stimulation were measured in vitro. MIF was assayed with a recently developed test system using the human monocytoid cell-line U937 as indicator cells in agarose microdroplets. Two-thirds of the 75 patients tested showed a defective DTH response to one or more of the microbial antigens; this contrasted to the findings in 25 healthy subjects, of whom over 90% showed a positive DTH reaction to any of the three antigens. PHA skin tests were entirely normal in both patients and healthy controls. Microbial antigen-specific LTT responses fluctuated considerably in time from strongly positive to negative and vice versa in healthy individuals as well as in patients. In general however, blastogenic responses in patients were comparable to or even higher than those of healthy persons. In the MIF assay, lymphocytes of all healthy individuals tested showed production of MIF upon stimulation with all three antigens; this again contrasted to two-thirds of the patients, whose lymphocytes showed a defective MIF production. Fluctuations of MIF-production in time could not be established and a very good correlation existed between the data obtained in the MIF assay and those of the DTH skin tests. These results indicate that apart from skin testing, the MIF assay seems to be the most suitable parameter to assess defects in T cell reactivity towards microbial antigens. These defects exist in two-thirds of our patients suffering from chronic purulent rhinosinusitis.