Anti B and T lymphocyte antibodies were sought in the serum of 22 rheumatoid arthritis patients from the Marseille area. No immunization against T lymphocytes was found. In contrast anti B lymphocyte activity was demonstrated in every diluted serum tested against a panel of 21 cells. In these sera, the complement dependent cytotoxicity was of medium intensity (lysis - 50 to 90 percent), antibody titers were high (medium 1/512) and a zone phenomenon was observed in 14 out of 22 non-diluted sera. As a rule, serum reactivity was slightly higher at 4 degrees C than at 22 degrees or 37 degrees C but at the latter temperature, it seemed to be more related to the target cells than to the serum tested. These anti B lymphocyte antibodies were predominantly IgMs and were not autoantibodies. Their specificity is still under study but already appears to be wide and not HLA related. These antibodies reacted with 9 to 100 percent of the panel cells and 85 percent of the sera recognized over 50 percent of the cells. Thus the antigenic determinants were probably allotypes or idiotypes of Fab fragments from surface immunoglobulins of a B lymphocyte subpopulation. Thus far, no relationship has been demonstrated between the presence or the titer of these antibodies and clinical, biological or genetic features of rheumatoid arthritis. In the future, this method should simplify the detection of Ig seric groups. It already allows detection of antiglobulin factors which could play an immunoregulatory role in rheumatoid arthritis.