One hundred and forty-two RA patients and 121 of their healthy same-sexed siblings were tested for antibodies (Abs) to Proteus mirabilis and Escherichia coli by indirect immunofluorescence. Eighty-five individuals had active RA (CRP greater than 10 mg/l) and 57 inactive RA. The anti-Proteus Ab titre in the active RA group was significantly higher than in the inactive RA and non-RA group (P less than 0.0001 in both cases). Anti-Proteus Abs were significantly elevated in 30 individuals with active RA compared with their healthy HLA-identical same-sexed siblings (P less than 0.001). In 42 HLA-DR4 positive and 15 HLA-DR4 negative active RA patients, the Ab titre was non-significantly higher in the former group. Anti-E. coli Abs were not significantly elevated in any of the groups. Longitudinal data on 36 RA patients demonstrated a significant positive correlation between changes in CRP and changes in anti-Proteus antibody titres (r = 0.52, P less than 0.001). These observations require an explanation.