Antibodies to proteus were determined by indirect immunofluorescence in 146 serum samples from patients with rheumatoid arthritis (RA). An autoantibody screen was performed in the same samples and in 52 of these antibody titres to the viruses influenza A, adenovirus, rubella, and parvovirus were determined. There was no significant correlation between proteus antibodies and any of the other antibodies tested. Dividing the samples into those from patients with active (C reactive protein > 10 mg/l) and inactive RA showed that the only antibodies to be significantly increased in active RA were the proteus antibodies. These observations suggest that the proteus antibody response in RA is specific.