The aim of this study was to analyse the in vitro response of human peripheral blood mononuclear cells to stimulation with killed Haemophilus influenzae strains of different capsular types, isolation sites and from cases with different forms of infections. The mean stimulatory index using 10(6) bacteria/well was 10, and 80 when 10(8) bacteria/well were used for stimulation. The mean+/-SD level was 13+/-4 ng/ml for interleukin (IL)-1beta, 128+/-73 ng/ml for IL-6, 203+/-122 ng/ml for IL-8, 3160+/-1220 pg/ml for IL-10, 29+/-40 pg/ml for IL-12, 2800+/-1790 pg/ml for tumour necrosis factor (TNF)-alpha and 4+/-7 ng/ml for interferon (IFN)-gamma, when stimulating cells with the lower dose of 10(6) bacteria/well. Using the higher bacterial dose, the levels of IL-1beta, TNF-alpha and IL-12 remained similar, whereas the IL-6, IL-8 and IL-10 levels were significantly lower, and IFN-gamma levels were significantly higher. Strains isolated from the bronchial tree induced significantly higher levels of IFN-gamma and significantly lower levels of IL-6, IL-8 and IL-10 than strains from other isolation sites. In conclusion, H. influenzae generated phagocyte-activating cytokines and an IL-10/IL-12 ratio that was 1090 times that described previously for Streptococcus pneumoniae.