Recent cDNA sequencing data has allowed the prediction of the entire amino acid sequences of complement components factor B and C2, the complement control proteins factor H and C4b-binding protein and a partial sequence for the Cab/C4b receptor CR1. These proteins all contain internal repeating units of approximately 60 amino acids, each repeating unit having a characteristic framework of highly conserved residues. The N-terminal Ba and CA portions of factor B and C2 both contain 3 repeating units and the chains of C4b-binding protein and factor H contain 8 and 20 repeating units, respectively, while the precise number of units in CR1 is not known yet. These structurally homologous complement proteins are also functionally related as they all interact with C3b and C4b during activation of the cascade. The repeating units also occur in the functionally unrelated proteins subcomponent C1r, β2-glycoprotein 1, blood clotting factor XIII and interleukin-2 receptor. In this review Ken Reid and his colleagues propose that this could be a general feature of a superfamily of structurally related proteins.