The composition of the obligate anaerobic intestinal flora of patients with rheumatoid arthritis (RA) differed from that of healthy subjects (HS). Total numbers of aerobes as well as anaerobic coccoid rods were found elevated when compared with HS. Eubacterium species were found in all stool samples of both groups; Bifidobacterium species were present in seven (RA) and eight (HS) out of 10 subjects. From the flora of two RA patients and two HS Eubacterium species were isolated and identified. Cell wall fragments from four E. aerofaciens strains (two from RA, two from HS) were tested for arthritis induction in rats. All four strains induced chronic arthritis which was histologically confirmed. We concluded that in the normal intestinal flora of RA patients Eubacterium species are present in high numbers (i.e. greater than 10(9)/g faeces); cell walls from isolated E. aerofaciens strains had arthropathic properties.