Urinary isolates of Proteus mirabilis, obtained from 49 RA patients and 44 healthy controls, were tested for susceptibility to antibiotics by the disc diffusion method. In addition, P. mirabilis isolates were also tested for proticine production and sensitivity (p/s) typing by the inhibition of growth of each test isolate against 13 reference strains of P. mirabilis. The P. mirabilis isolates from both RA patients and healthy controls were highly susceptible to norfloxacin, ciprofloxacin and trimethoprim, but less to minocycline. The urine of RA patients contained fewer different types of P. mirabilis strains than those isolated from healthy controls. All of the strains found in the RA patients were proticine producers (P < 0.001), mostly of proticine 3 (P < 0.005). The presence of such strains provides evidence of a sub-clinical upper urinary tract infection with P. mirabilis in some RA patients. Therapeutic intervention in RA with relevant antibiotics requires evaluation.