Using lactose as an inducer, recombinant human interleukin-2 (rhIL-2) was synthesized with an N-terminus fusion partner, G3 (three tandem-arranged glucagon peptides) in fed-batch cultures at high cell concentration (60-90 g l(-1)) of Escherichia coli BL21(DE3) [pT7-G3IL2]. With batch additions of lactose (4 x 13.5 g), the fusion rhIL-2 was synthesized up to 9.3 g l(-1). However, if all the lactose (54 g) was added at once to the culture, synthesized fusion rhIL-2 decreased to 5.4 g l(-1) with a decreased cell growth rate. A statistical optimization of the production medium containing glucose, yeast extract, and lactose led to fusion rhIL-2 being produced at > 9 g l(-1).