The effect of lac operon in the induction using lactose for the expression of periplasmic human interferon-α2b (PrIFN-α2b) was studied in shake flask culture. Escherichia coli strains Rosetta2 (DE3) [R2 (DE3)] containing the lac operon and Rosetta-gami2 (DE3) [RG2 (DE3)] containing the deletion of entire lac operon with high level of lac repressor were used. R2 (DE3) over-expressed PrIFN-α2b at substantial levels (270-380 μg/L) in lactose-induced media. In spite of the deletion of lac operon in RG2 (DE3), the cells exposed to lactose produced PrIFN-α2b albeit in less quantity (18-20 μg/L). Under similar conditions, the percentage of IFN-α2b translocated into periplasm for cells induced with lactose was 43-57 and 15-30% in R2 (DE3) and RG2 (DE3), respectively. The PrIFN-α2b expressed by RG2 (DE3) grown in control medium and Terrific broth was 290.3 and 134.7 μg/L, respectively. The basal expression levels obtained in R2 (DE3) strain were 10-fold higher than those obtained in RG2 (DE3) strain. The target proteins expressed in uninduced state did not affect the growth, indicating that IFN-α2b was non-toxic to the bacterial cells. Equivalent level of PrIFN-α2b expression was obtained in RG2 (DE3) induced by IPTG and in R2 (DE3) induced by lactose.