A new E. coli expression system, the pHCE-IIB vector, using a constitutively expressing HCE promoter derived upstream from the D-amino acid aminotransferase (D-AAT) gene of Geobacillus toebii, was developed for the high-level expression of foreign proteins without induction. The expression efficiency of recombinant human tumor necrosis factor-alpha (rhTNFalpha) using pHCE-IIB was compared with that of the pET14B vector under the control of a T7 promoter induced by IPTG. The total amount of rhTNFalpha produced by the pHCE system was approximately twice that produced by the pET system under optimal fermentation conditions thereby demonstrating the convenience and economical advantage of the new constitutive pHCE-IIB expression vector over the conventional pET expression vector.