15 patients with active RA were observed over a 3-month period after starting chloroquine treatment. Clinical condition, plasma levels of chloroquine and levels of 15 individual plasma proteins were checked monthly. Nine patients responded favourably to therapy, 6 failed to respond. The responders had lower initial CRP, orosomucoid and ceruloplasmin levels, whereas their IgA and IgM levels were slightly elevated. Significant reductions in the levels of CRP, haptoglobin, orosomucoid, fibrinogen and ceruloplasmin occurred in the responder group of patients. Alfa1-antitrypsin, antichymotrypsin C3 and C4 levels within the normal range were frequently encountered despite other clear-cut signs of activity. The chloroquine levels did not differ between responders and non-responders, the mean concentrations being 1.04 and 1.6 micromol/l respectively. This study has also demonstrated that in selected cases, despite active joint disease, all acute phase proteins may be normal. Finally, it was obvious that chloroquine, even when inducing remission, only brought about a partial normalization of the plasma protein pattern.