In most normal human sera the addition of penicillamine to a final concentration of 0-2 mmol/l and subsequent dialysis caused a slight reduction in serum haemolytic complement (CH50). At 200 mmol/l, CH50 activity was no longer demonstrable. Even high concentrations of penicillamine were needed to inhibit the ability of immunoglobulin to fix complement to preformed or forming immune complexes. This indicated that the reduction of CH50 observed in serum was due to an effect on the complement factors. In vivo, a dose of 240 mg penicillamine caused a slight transient reduction in CH50 in rabbit serum, while 1000 mg penicillamine had no effect on serum CH50 in patients with rheumatoid arthritis. In arthritis patients there was, however, some evidence for removal of complement deposits in synovial tissue during penicillamine treatment. Since it is theoretically possible that concentrations high enough to cause reduction of complement activity can be achieved locally in synovial tissue, the effect on complement may be one of the mechanisms by which penicillamine exerts its effect in rheumatoid arthritis.