The distribution of Proteus antibody levels was compared in English, Norwegian and Spanish patients with rheumatoid arthritis (RA). Using an indirect immunofluorescence method, the IgG antibody titre against Proteus mirabilis was measured in the sera of 27 English, 53 Norwegian and 34 Spanish patients with RA and divided into active and inactive disease groups according to the serum C-reactive protein (CRP) level (> or = 10 mg/l). Serum samples were also collected from 25 English, 30 Norwegian and 14 Spanish healthy individuals who served as controls. The levels of Proteus IgG antibodies were significantly higher in the sera of active RA patients (p<0.001) when compared with the corresponding healthy controls, whether these groups belonged to the English, Norwegian or Spanish populations. Furthermore, active RA patients from each country showed significantly higher levels of Proteus antibodies when compared with inactive English (p<0.01), Norwegian (p<0.001) or Spanish (p<0.001) RA patients. Finally, a significant correlation was observed between Proteus IgG antibody levels and the CRP concentrations in RA patients whether each population was tested individually or all together (p<0.001). The increased levels of Proteus antibodies in RA patients from three different European countries support the concept of a possible aetiopathogenetic role for Proteus microorganisms in the development of RA.