We previously reported four different cDNA clones encoding human dopamine-beta-hydroxylase (Kobayashi et al., Nucl. Acids Res., 17 (1989) 1089-1102). These clones were different in a 3' untranslated region (types A and B) and/or in 6 nucleotides in mRNAs. The difference at nucleotide 910 caused an amino acid change between Ala (A) and Ser (S) at amino acid residue 304 (DBH/A and DBH/S). We succeeded in expressing both of DBH/A and DBH/S of type A cDNAs in COS cell. Both of the expressed proteins showed enzyme activities and immunoreactivities. The two proteins had similar kinetic constants, but had different homospecific activities (activities per enzyme protein); the homospecific activity of human DBH/S was low, approximately one thirteenth that of human DBH/A.