The importance of T-cell receptor (TcR) V beta gene haplotype and complement component C5 deficiency for the resistance to collagen-induced arthritis (CIA) in the SWR mouse was studied. Firstly, the immune responses against heterologous and autologous type II collagen (CII) were compared between SWR mice and arthritis-susceptible and major histocompatibility complex (MHC)-identical DBA/1 mice. Secondly, F1 and F2 crosses were made between the two strains and studied for arthritis development, V beta gene usage and C5 presence. After immunization with heterologous rat CII, both strains reacted with a strong autoantibody response. Immunization with mouse CII, on the other hand, gave a much lower response in both strains, with DBA/1 mice having the strongest response. A collection of B-cell hybridomas was created from the draining lymph nodes of SWR mice 9 days after immunization with rat CII. This hybridoma collection showed similarity to previous data from the DBA/1 mouse, by its high frequency of B cells producing IgG specific for CII and with a high degree of cross-reactivity with autologous mouse CII. After immunization with heterologous CII, both T cells specific for heterologous CII as well as T cells cross-reacting with autologous CII could be demonstrated. F1 crosses between SWR and DBA/1 were relatively resistant to CIA (8%), while in the F2 generation 72% of the mice developed arthritis. No correlation between V beta haplotype or C5 and development of arthritis in the F2 mice was found. It is concluded that the resistance to CIA in the SWR mouse is not related to either V beta haplotype or C5 deficiency. Instead we postulate the involvement of two or more genes which are important for the induction and maintenance of tolerance to own CII.