In a survey of 89 RA patients, carried out under code, Proteus mirabilis was isolated from the urine of 63% (47/75) of female (P < 0.001) and 50% (7/14) of male patients (P < 0.001), compared to a frequency of isolation in healthy women of 32% (38/119) and 11% (13/115) in healthy men. There was no significant difference in isolation rates between 37 non-RA patients and healthy controls. Sera from 20 patients with RA and 20 healthy controls were tested against P. mirabilis and Escherichia coli by an enzyme-linked immunosorbent assay. Antibodies against P. mirabilis but not to E. coli were significantly higher in the RA patients than in healthy controls (P < 0.001). Furthermore, a positive correlation was found between high anti-Proteus antibody levels in serum samples and the number of Proteus colony-forming units obtained from urine specimens of the 20 RA patients (r = +0.714, P < 0.001). These results support the suggestion of an aetiopathogenic role for P. mirabilis in RA.