The purification of type II collagen, for the detection of anti-type II collagen antibodies by ELISA procedures, involves removal of proteoglycans by guanidine-HCl, followed by pepsin solubilisation and salt fractionation. However, type II collagen purified in this way may contain contaminants, despite the apparent purity on SDS-polyacrylamide gels. In this paper we demonstrate how additional purification by DEAE chromatography reduces the degree of background binding in the type II collagen ELISA, leading to an increase in disease specificity. The contaminants included proteoglycan and bound serum IgG from both rheumatoid arthritis (RA) patients and healthy controls in ELISA. Furthermore, positive correlations were observed in the sera (n = 24) between degree of reactivity to the contaminants and to (1) purified proteoglycan (r = 0.50, P = 0.01) and (2) pepsin (r = 0.65, P = 0.001). Thus, inadequate purification of type II collagen produces false positive reactions in the collagen ELISA and gives rise to a high background. A lack of specificity has been frequently associated with this assay.