Cyanogen bromide cleavage of reductively alkylated homogeneous rat liver dihydropteridine reductase afforded several peptide fragments identifiable by polyacrylamide electrophoresis of which 6 (CB-1 to CB-6) could be individually isolated by C8 reverse phase HPLC. Each was characterised by N-terminal amino acid analysis and sequence information was derived for CB-1, CB-4 and CB-6. The blocked N-terminal of the holoenzyme was identified as pyroglutamate and the C-terminal sequence was obtained by sequential degradation.