Much evidence suggests that IgA production in vivo and in vitro is enhanced in patients with IgA nephropathy (IgAN). We have demonstrated glomerular deposition of the outer membranes of Haemophilus parainfluenzae (HP) antigens (OMHP) and the presence of HP-specific IgA in the serum of patients with IgAN. In this study, we investigated the production of IgA and several cytokines by tonsillar mononuclear cells (TMC) from IgAN patients induced by stimulation with OMHP. The spontaneous production of total IgA and TGF-beta by TMC from IgAN patients was higher than that by TMC from patients with chronic tonsillitis (CT) (P < 0.05). Stimulation with OMHP in vitro enhanced the production of HP-specific IgA by TMC from IgAN patients (P < 0.01), but not by TMC from CT patients. OMHP stimulation also enhanced the production of TGF-beta and IL-10 by TMC from IgAN patients (P < 0.001). These results suggest that the infection of HP in the tonsil may be involved in the etiology of IgAN.