The effect of D-penicillamine on experimental allergic arthritis in rabbits (16 with arthritis and 15 controls) was investigated. After 40 days of treatment with D-penicillamine (intravenous) the arthritis had receded to a large extent. At the same time a maturation inhibition of the hematopoietic marrow had occurred in all the treated arthritic and control animals. The results suggest that D-penicillamine affects fast proliferating cells, to which the precursors of immunologically active cells in the synovial membrane also belong.