In the present investigation, the incidence, day of onset and severity of type II collagen (IIC)-induced polyarthritis (CA) in allogeneic bone marrow chimaeras were compared. These chimaeras were prepared by using reciprocal combinations between DBA/1J (H-2q, Igh-1c, Hc1), which are highly susceptible to CA, and SWR/J (H-2q, Igh-1c, Hc0), which are CA resistant, strains of mice. The four kinds of chimaeras, [SWR----DBA/1], [DBA/1----DBA/1], [DBA/1----SWR] and [SWR----SWR], were shown to be full chimaeras. [SWR----DBA/1] and [DBA/1----DBA/1] chimaeras showed serum C5 antigens and most of these mice developed CA, even though severity of CA in [SWR----DBA/1] chimaeras was less than that in [DBA/1----DBA/1] syngeneic chimaeras. By contrast, [DBA/1----SWR] and [SWR----SWR] chimaeras completely lacked serum C5 and did not develop CA at all. However, most of these chimaeras produced considerable total IgG and IgG2a antibodies to IIC in the sera after immunization with the antigens, although differences in the serum levels of IgG2a antibodies were seen between [SWR----DBA/1] and [DBA/1----SWR] allogeneic chimaeras. The present results demonstrate directly a crucial role of complement cascade in the induction and development of CA and reveal that lymphoid cells of SWR mice potentially have the ability to develop CA. Differences in severity of the CA observed in some of these chimaeras, however, could not be readily explained by differences in C5 concentrations alone.