The K/BxN murine model of rheumatoid arthritis (RA) is dependent on the specificity of the KRN § g -TCR, to recognize glucose-6-phosphate-isomerase (GPI) on the NOD MHC class II A g7 allele and production of GPI-specific autoantibodies. Transfer of K/BxN serum into MHCunrelated and lymphocyte-deficient mice induces RA. To investigate whether K/BxN seruminduced RA involves complement activation and/or the complement receptors (CR) 1 and 2, we analyzed the role of complement C4 and of CR1 and CR2. For this purpose we used C4 -/- mice impaired in the classical and the lectin complement pathways; Cr2 -/-mice lacking CR1 and CR2 and, as control strains, BALB/c, C57BL/6, KRN and NOD. RA was assessed by calliper measurement of ankle thickness, clinical index and joint histology. We found that all mouse strains except NOD developed RA. The lack of protection in C4 -/-mice suggests that antibody-mediated RA is independent of the classical as well as the lectin complement pathways and the split complement product C4b. The lack of protection in Cr2 -/-mice suggests that absence of CR1 had no significant affect, considering its role in immune complex clearance, inhibition of C3 and C5 convertase and as receptor for C3b/C4b. Also, CR2 lacks a role in disease as analyzed here, in its possible functions as receptor for C3dg, germinal center reaction and activation of alternative pathway on binding iC3. Hence we conclude that the transmission of K/BxN serum-induced RA is independent of the classical and the lectin complement pathways and CR1 and CR2. The crucial role of complement C5, while neither classical nor lectin pathway is necessary, indicates that the alternative complement pathway may have a role in the K/BxN serum-induced RA model.