Accumulating amino acid sequence data have made it increasingly evident that many essential complement proteins have potentially modifiable lysine residues in putative critical functional regions. Evidence is now presented that glucose is covalently attached to lysine-266 of purified human complement Factor B as a result of glycation. Purified B was treated with NaB3H4, which reduces such bound glucose to a mixture of radiolabelled hexitols. Amino acid analysis revealed the expected radiolabelled hexitol-lysine epimers. In addition, fluorography of dried gels resolving the major high-molecular-mass h.p.l.c.-fractionated CNBr-cleavage peptides of NaB3H4-reduced B indicated that this radioactivity was specifically associated with the 15 kDa fragment derived from the N-terminal region of fragment Bb. Amino acid sequence analysis suggested that the C-terminal lysine (residue 266 of B) of the N-terminal Lys-Lys doublet of this peptide is preferentially modified. If such glycation can subsequently be shown to occur in vivo, then perhaps this modification might also be found to affect the functional activity of B and offer a potential explanation for some of the immunopathological complications of diseases exposing key plasma proteins, such as this active-site-containing proteinase of the multimeric alternative-complement-pathway C3/C5 convertases, to long-term high concentrations of glucose, such as the decreased resistance to infection and impaired chemotaxis and phagocytosis characteristic of diabetes.