We have fused the epidermal growth factor (EGF) to the amino terminus of Pseudomonas exotoxin A (PE) to create a cytotoxic agent, designated EGF-PE, which preferentially kills EGF-receptor-bearing cells. In this study, we analyzed the effect of the Ia domain, the binding domain, of PE on the cytotoxicity of ECF-PE towards EGF-receptor-bearing cells and tried to develop a more potent EGF-receptor-targeting toxin. EGF-PE molecules with sequential deletions at the amino terminus of PE were constructed and expressed in E. coli strain BL21(DE3). The cytotoxicity of these chimeric toxins was then examined. Our results show that the amino-terminal and carboxy-terminal regions of the Ia domain of PE are important for the cytotoxicity of a PE-based targeting toxin. To design a more potent PE-based EGF-receptor-targeting toxin, a chimeric toxin, named EGF-PE(Delta 34-220), which had most of the Ia domain deleted but retained amino acid residues 1-33 and 221-252 of this domain, was constructed. EGF-PE(Delta 34-220) has EGF-receptor-binding activity but does not show PE-receptor-binding activity and is mildly cytotoxic to EGF-receptor-deficient NR6 cells. As expected, EGF-PE(Delta 34-220) is a more potent cytotoxic agent towards EGF-receptor-bearing cells than EGF-PE(Delta 1-252), where the entire Ia domain of PE was deleted. In addition, EGF-PE(Delta 34-220) was shown to be extremely cytotoxic to EGF-receptor-bearing cancer cells, such as A431, CE81T/VGH, and KB-3-1 cells. We also found that EGF-PE(Delta 34-220) was highly expressed in BL21(DE3) and could be easily purified by urea extraction. Thus, EGF-PE(Delta 34-220) can be a useful cytotoxic agent towards EGF-receptor-bearing cells.