Experimental inflammation in copper (Cu)-deficient rats is greater than that induced in controls eating normal diet. Cu-supplementation of the Cu-deficient diet results in a reduced swelling, down to normal levels. Injection of the naturally occurring acute phase reactant, ceruloplasmin (Cp) a Cu-bearing serum protein, also results in reduction of experimental inflammation. Since a rise in serum Cp occurs in normal pregnancy this protective anti-inflammatory action of Cp is proposed as an explanation for the widely-observed phenomenon of spontaneous control of rheumatoid arthritis in pregnancy.