The inhibitory effects of D-penicillamine (D-Pen) on lymphocyte activation by PHA are found to be dose-dependent, showing significant effects above a concentration of 50 microgram/ml. The cells show a diminished sensitivity to D-Pen added after 24 hours in culture, although the dose-response relationship is maintained. L-cysteine is usually found to have an insignificant effect on PHA stimulation, although enhancement and inhibition are also sometimes observed. The supplementation of culture medium with L-cysteine abolishes the inhibitory effect of D-Pen, whilst copper sulphate synergistically inhibits PHA-induced transformation. The balance between the availability of naturally occurring thiols, copper ions and administered D-Pen may be critical in determining the eventual effect of the drug on the lymphoid system. Lymphocyte suppressive soluble factors appear in the sera of patients with rheumatoid arthritis and are modified by treatment with D-Pen. Although our experiments suggest possible modes of action of the drug on the immune system as a therapeutic agent as well as a potent inducer of autoimmunity in its own right, further work is required to delineate its precise mechanism of action.