The major objective of this study is to identify fed-batch culture conditions optimal for the production of human interleukin-6 (hIL-6) in a soluble form. Five different expression vectors were constructed for the expression of hIL-6 and hIL-6s fused with NusA, maltose binding protein (MBP), thioredoxin (Trx) or ubiquitin (Ubi). A series of flask cultures were conducted in LB medium at 37 degrees C. The intact hIL-6 was expressed mostly in the form of inclusion body. More than 95% of the hIL-6 fused with NusA (NusA/hIL-6) and about 90% of MBP/hIL-6 were expressed in a soluble form, whereas Trx/hIL-6 and Ubi/hIL-6 were expressed mostly in the form of inclusion body. Based on this result, NusA was selected as the fusion partner for the production of hIL-6 in the subsequent experiments. A series of pH-stat fed-batch cultures of an E. coli BL21(DE3) transformed with a NusA/hIL-6 expression vector were conducted in a bioreactor with a working volume of about 3 L. As the amount of nitrogen source was increased in the feeding medium, more soluble NusA/hIL-6 was produced, while the total amount was not significantly changed. Under the best conditions tested, about 90% of NusA/hIL-6 was produced in the soluble form. In this case, the concentration of soluble NusA/hIL-6 was 7.5 g/L with a volumetric productivity of 0.43 g/L-h.