The role of cellular immunity in collagen arthritis was investigated with congenitally athymic nude rats (rnu/rnu) and their heterozygous littermates (rnu/+). Immunization with type II collagen induced polyarthritis and definite immunity to type II collagen in rnu/+ rats, whereas rnu/rnu rats did not develop arthritis or immunity to collagen. An additional study demonstrated that collagen arthritis could be passively transferred with a serum concentrate from arthritic Sprague-Dawley rats to naive rnu/rnu rats as well as to rnu/+ rats. Histopathologically, the passively transferred arthritis in rnu/rnu rats resembled that in rnu/+ rats. Despite no difference in clearance of anti-type II collagen antibody after transfer between them, the passively transferred arthritis in rnu/rnu rats was significantly enhanced and prolonged in comparison with that in rnu/+ rats. These results indicate that arthritis may be inducible by humoral immunity in the absence of functional T cells and also suggest that anti-type II collagen-antibody is not the sole regulatory factor and that the suppressor cell system might regulate the clinical course of the disease.