It has been suggested that autoimmunity to the proteoglycan (PG) component of cartilage plays a major role in the etiology of adjuvant arthritis (AA), which occurs in rats, but not in mice, after injection of CFA. In order to more directly investigate this role, bovine and human cartilage PG were used to modulate AA, and immunity to PG was assessed. Immunization of rats or mice with PG by itself does not induce arthritis. However, in rats, a single i.v. injection of soluble PG, given 1 wk before injection of CFA, results in a significant increase in incidence and severity of the arthritis induced. Rats injected with CFA have both antibody and delayed type hypersensitivity (DTH) to PG. Upon pretreatment of rats with PG i.v., both DTH and antibody titers to PG are increased. Rats immunized with PG in IFA have high titers of anti-PG and strong DTH to PG, which are also enhanced by pretreatment with PG i.v., although none of these animals develops arthritis. In contrast to these findings in rats, when mice are pretreated with PG i.v., DTH to PG induced by injection of CFA is lower, whereas anti-PG titers are higher than in unpretreated controls. The results presented here show that, in rats, i.v. injection of PG synergizes with CFA in the induction of AA, and enhances both humoral and cellular immunity to PG. The findings support the hypothesis that immunity to PG is of importance in AA, although under the conditions of these experiments immunity induced by PG alone is clearly not sufficient for the induction of arthritis.