In 79 RA patients treated with Bucillamine (Bu) we monitored IgG, A, M and total protein concentration x gamma-globulin% (Ig) before and after Bu. All of these four were lowered after Bu in both groups with and without adverse reaction. In the group with adverse reactions the serum level of IgG, A and Ig was significantly lower after Bu treatment than in the group without adverse reactions. The decreases of IgG and IgA were statistically significantly greater in the group with adverse reactions than those in the group without adverse reaction. The serum level of IgM after Bu in the effective group was significantly lower than that in the non-effective group. We also examined lymphocyte stimulation test (LST) in 44 RA patients treated with Bu. In the effective group Bu inhibited lymphocyte proliferative response to PPD more significantly than in the non-effective group. Bu also inhibited lymphocyte proliferative response after stimulation with PPD in the non-effective group doseresponsively. We concluded that the considerable decreases of IgG and IgA might correlate with the adverse reactions of Bu. The decrease of IgM and inhibition of LST with Bu might correlate with the efficacy of Bu.