SummaryTwenty patients with classical or definite rheumatoid arthritis (R.A.) were treated with D penicillamine, six were withdrawn from the study between the 4th and 18th weeks of treatment: 2 for lack of improvement, 2 for thrombocytopenia and 2 for proteinuria.Fourteen patients with a good response to a 31 week long treatement and without untoward effects on renal, liver and haematological functions were selected for this study. The serum hemolytic complement (CH50), the complement components C3, C4 and C5; the serum levels of immunoglobulins IgA, IgM and IgG, together with other biological and clinical parameters were measured during the course of treatment.The study of these patients revealed a significant decrease of IgA and IgG levels and an improvement of inflammatory parameters correlating with the clinical evolution. D-pcnicillamine had no effect on complement components C3, C4 and C5 while an increase of CH50 from the 7th week onwards, after an initial period of decrease was observed.