SWR mice are resistant to collagen-induced arthritis but produce antibodies to type II collagen. To determine if these antibodies have arthritogenic potential, serum from collagen-immunized mice was concentrated and passively transferred to DBA/1 mice. The recipients developed severe arthritis within 72 hours. To evaluate the role of complement, SWR mice were bred with congenic inbred B10.D2/oSn (complement deficient) and B10.D2/nSn (complement normal) mice. Collagen-immunized (SWR x B10.D2/nSn)F1 mice had high levels of C5 and were susceptible to arthritis, while (SWR x B10.D2/oSn)F1 mice were deficient in C5 and were resistant to arthritis.