We have previously reported that a variety of solid human tumor cell lines express a large number of receptors for interleukin- 13 (IL-13). These receptors could be targeted with a chimeric fusion protein consisting of human IL-13 and a truncated form of Pseudomonas exotoxin (PE). We describe here optimization of critical steps involved in high yield expression of two recombinant chimeric fusion proteins for obtaining highly purified and biologically active cytotoxins in Escherichia coli. The chimeric constructs of human IL-13 and two 38 kDa truncated PEs: (i) PE38 and (ii) PE38QQR. (three lysine residues in PE38 at 590, 606. and 613 substituted with two glutamine and one arginine) were used for protein expression in pET prokaryotic expression vector system with kanamycin as a selection antibiotic. Our results suggest that fresh transformation of E coli and induction by isopropyl-beta-D-thiogalactopyranosid,(IPTG) for 6 h resulted in maximum protein expression. To further improve the yield. we used a genetically modified E. coli strain. BL21 (DE3)pLysS. which carries a plasmid for lysozyme with a weak promoter that inhibits T7 RNA polymerase and minimizes protein production in the absence of IPTG. Use of this strain eliminated the need for lysozyme digestion of the induced bacteria to release inclusion bodies, which resulted in expression of purer protein as compared to the conventional BL21(DE3) strain. Additional protocol optimizations included 16 h solubilization of inclusion bodies, constitution of refolding buffer. and timing of dialysis. These proteins were finally purified by Q-Sepharose, mono-Q, and gel filtration chromatography. Between 14-22 and 21-28mg highly purified and biologically active protein was obtained from 1 L of BL21 (DE3) and BL21 (DE3) pLysS bacteria culture, respectively. As IL-13R targeting for brain tumor therapy offers an exciting treatment option. optimization of production of IL-13PE will enhance production of clinical grade material for Phase III clinical trials. Published by Elsevier Inc.