In order to study the periplasmic expression of human growth hormone (hGH) in Escherichia coli, the related cDNA was inserted in two expression plasmids carrying pelB signal peptide, one with lac bacterial promoter and the other with a bacteriophage T7-based promoter. The recombinant plasmids were moved to TG1 and BL21 strains of E. coli, respectively. To induce the expression systems, IPTG and its natural analog, lactose, were examined. Results show the over-expression of recombinant hGH (rhGH) in the T7-based system which is much higher than that of the lac-regulated system. In both systems, a fraction of the hGH, produced by recombinant bacteria, remains in the cytoplasm as pre-protein and the rest is transferred to the periplasmic space as mature protein. Any decline in the expression level did not lead to a complete processing and transport of mature hGH to the periplasmic space of the bacteria. It is suggested that, for an efficient expression of rhGH in the periplasmic spaceof E. coli, a combined approach including application of a suitable signal peptide, solubility of the over-expressed protein in cytoplasm in addition to the optimization of the bacterial growth and inducing conditions should be considered.