The effects of D-penicillamine on T-lymphocytes (rosette forming cells) and B-lymphocytes (surface IgG- and surface IgM-bearing cells) in peripheral blood of 13 patients with classical or definite rheumatoid arthritis were investigated at 4 weeks, 12 weeks and 24 weeks of treatment. At the same time, the determination of the concentrations of serum immunoglobulins and RA tests were carried out, and the rheumatoid activity index was calculated. The administration of D-pencillamine caused a gradual increase in the percentage of T-lymphocytes and a concomitant decrease in the percentage of B-lymphocytes with time. It was found that the tendency toward reduction of the percentage of surface IgM-bearing lymphocytes was more remarkable and accelerated than that of surface IgG-bearing lymphocytes. A decrease in the concentrations of serum IgM and IgG was observed in association with these findings. The agglutination in RA-test became significantly weaker in about half of the cases. Also the rheumatoid activity index became gradually smaller after treatment. The marked decrease in the percentage of B-lymphocytes and in the concentration of serum immunoglobulins may be related to the inhibition of the production of abnormal immunoglobulins.